Motion vector calculation method, picture coding method, picture decoding method, motion vector calculation apparatus, and picture coding and decoding apparatus

ABSTRACT

A motion vector calculation method which attains a higher compression rate, includes: a selection step of selecting one of at least one reference motion vector of a reference block; and a calculation step of calculating a motion vector of a current block to be processed, using the one reference motion vector selected in the selection step, and in the selection step, when the reference block has two reference motion vectors, one of the two reference motion vectors is selected based on whether the reference block is located before or after the current block in display time order, and when the reference block has only one reference motion vector, the one reference motion vector is selected.

TECHNICAL FIELD

The present invention relates to a method of calculating a motionvector, a picture coding method and a picture decoding method which usethe motion vector, and so on.

BACKGROUND ART

In coding processing of moving pictures, a quantity of information isgenerally reduced using redundancy of the moving pictures in spatial andtemporal directions. Here, a general method using the redundancy in thespatial direction is represented by the transformation into frequencydomain while a general method using the redundancy in the temporaldirection is represented by inter-picture prediction (hereinafterreferred to as inter prediction). In a coding process using the interprediction (an inter prediction coding process), when coding a certainpicture, a coded picture located before or after the current picture tobe coded in display time order is used as a reference picture.Subsequently, a motion vector of the current picture with respect to thereference picture is estimated, and a difference is calculated betweenimage data of the current picture and prediction picture data resultingfrom motion compensation based on the motion vector, to remove theredundancy in the temporal direction.

In the moving picture coding scheme called H. 264, which has alreadybeen standardized, three types of pictures: I-picture, P-picture, andB-picture, are used to reduce the quantity of information. The I-pictureis a picture on which no inter prediction coding process is performed,that is, on which a coding process using intra-picture prediction(hereinafter referred to as intra prediction) is performed. TheP-picture is a picture on which the inter prediction coding process isperformed with reference to one coded picture located before or afterthe current picture in display time order. The B-picture is a picture onwhich the inter prediction coding process is performed with reference totwo coded pictures located before or after the current picture indisplay time order.

Furthermore, in the moving picture coding scheme called H. 264, a codingmode which is referred to as temporal direct can be selected to derive amotion vector in coding the B-picture (see Non Patent Literature 1, forexample). The inter prediction coding process in temporal direct isdescribed with reference to FIG. 1.

FIG. 1 illustrates an inter prediction coding process in temporal directand a method of calculating a motion vector.

As shown in FIG. 1, a block (to be processed) Ba of a picture (to becoded) B2 is coded in the inter prediction coding process in temporaldirect. In this case, a motion vector “a” is used which has been used tocode a block Bb, co-located with the block Ba, in a picture P3 servingas a reference picture located after the picture B2. The motion vector“a” is a motion vector which has been used to code the block Bb andrefers to a picture (a reference picture) P1. Here, two motion vectors“b” and “c” parallel to the motion vector “a” are calculated for theblock Ba. Specifically, a block, indicated by the motion vector “b”,included in the reference picture P1 located before the block Ba indisplay time order and a block, indicated by the motion vector “c”,included in the reference picture P3 located after the block Ba indisplay time order are obtained, and using bi-directional predictionwith reference to the obtained blocks, the block Ba is coded. It is tobe noted that the motion vector to be used in coding the block Ba is themotion vector “b” directed forward to indicate the reference picture P1and the motion vector “c” directed backward to indicate the referencepicture P3.

CITATION LIST Non Patent Literature

[NPL 1]

ITU-T H.264 03/2010

SUMMARY OF INVENTION Technical Problem

However, in the conventional temporal direct, the motion vector to beused in the temporal direct, that is, the motion vector to be used incalculating a motion vector of a current block to be processed, is amotion vector of a reference picture (specifically, a reference block)located after the current block in display time order and limited to amotion vector directed forward in display time order.

Such a limitation of the motion vector to be used in the temporal directcauses problems of making it difficult to calculate the motion vectormost suitable for the current block, which leads to a decreasedcompression rate.

Thus, the present invention has an object to solve the above problems,and the object is to provide a motion vector calculation method, apicture coding method, a picture decoding method, and so on, whichderive the motion vector most suitable for the current block and attaina higher compression rate.

Solution to Problem

In order to achieve the above object, a motion vector calculation methodaccording to an aspect of the present invention is a motion vectorcalculation method of calculating a motion vector of a current block tobe processed that is included in a moving picture, the motion vectorcalculation method comprising: a selection step of selecting one of atleast one reference motion vector of a reference block; and acalculation step of calculating the motion vector of the current blockusing the one reference motion vector selected in the selection step,wherein, in the selection step, when the reference block has tworeference motion vectors, one of the two reference motion vectors isselected based on whether the reference block is located before or afterthe current block in display time order, and when the reference blockhas only one reference motion vector, the one reference motion vector isselected.

With this, one of two reference motion vectors is selected based onwhether the reference block is located before or after the current blockin display time order. For example, the reference block is a co-locatedblock, and the current block is a block to be coded or a block to bedecoded. Furthermore, the reference motion vector is a motion vectorused to code or decode the reference block. Thus, in the motion vectorcalculation method according to an aspect of the present invention, evenwhen the reference block has two reference motion vectors, a suitablereference motion vector can be selected according to a position of thereference block, and, for example, scaling the selected reference motionvector allows calculation or derivation of the most suitable motionvector for the current block. As a result, the compression rate of thecurrent block can increase.

Furthermore, it may be that in the selection step, in the case where thereference block has, as the two reference motion vectors, a forwardreference motion vector directed forward and a backward reference motionvector directed backward, the forward reference motion vector isselected from among the two reference motion vectors when the referenceblock is located after the current block, and the backward referencemotion vector is selected from among the two reference motion vectorswhen the reference block is located before the current block.

With this, a suitable reference motion vector can be reliably selected.

In order to achieve the above object, a motion vector calculation methodaccording to another aspect of the present invention is a motion vectorcalculation method of calculating a motion vector of a current block tobe processed that is included in a moving picture, the motion vectorcalculation method comprising: a selection step of selecting one of atleast one reference motion vector of a reference block; and acalculation step of calculating the motion vector of the current blockusing the one reference motion vector selected in the selection step,wherein, in the selection step, when the reference block has tworeference motion vectors, one of the two reference motion vectors isselected based on a temporal distance between the reference block and apicture indicated by each of the two reference motion vectors, and whenthe reference block has only one reference motion vector, the onereference motion vector is selected.

With this, one of the two reference motion vectors is selected based ontemporal distances between the reference block and the respectivepictures indicated by the two reference motion vectors. Thus, in themotion vector calculation method according to another aspect of thepresent invention, even when the reference block has two referencemotion vectors, a suitable reference motion vector can be selectedaccording to a temporal distance between pictures, and, for example,scaling the selected reference motion vector allows calculation orderivation of the most suitable motion vector for the current block. Asa result, the compression rate of the current block can increase.

In order to achieve the above object, a motion vector calculation methodaccording to another aspect of the present invention is a motion vectorcalculation method of calculating a motion vector of a current block tobe processed that is included in a moving picture, the motion vectorcalculation method comprising: a selection step of selecting one of atleast one reference motion vector of a reference block; and acalculation step of calculating the motion vector of the current blockusing the one reference motion vector selected in the selection step,wherein, in the selection step, when the reference block has tworeference motion vectors, one of the two reference motion vectors isselected based on a magnitude of each of the two reference motionvectors, and when the reference block has only one reference motionvector, the one reference motion vector is selected.

With this, one of the two reference motion vectors is selected based onrespective magnitudes of the two reference motion vectors. Thus, in themotion vector calculation method according to another aspect of thepresent invention, even when the reference block has two referencemotion vectors, a suitable reference motion vector can be selectedaccording to magnitudes of the two reference motion vectors, and, forexample, scaling the selected reference motion vector allows calculationor derivation of the most suitable motion vector for the current block.As a result, the compression rate of the current block can increase.

Furthermore, in order to achieve the above object, a picture codingmethod according to an aspect of the present invention is a picturecoding method of coding a moving picture, comprising: the selection stepand the calculation step in the motion vector calculation methodaccording to one of the above aspects of the present invention; and acoding step of coding the current block using the motion vectorcalculated in the calculation step.

With this, even when the reference block has two reference motionvectors, the current block can be coded using the most suitable motionvector calculated for the current block, which allows an increase in thecompression rate.

Furthermore, the picture coding method may comprise: a determinationstep of determining, as the reference block, one of a block locatedbefore the current block in display time order and a block located afterthe current block in display time order; a generation step of generatinga position flag indicating whether the reference block determined in thedetermination step is located before or after the current block; and anaddition step of adding the position flag generated in the generationstep, to a picture including the current block coded in the coding step.

With this, since a picture including the current coded block has aposition flag, the picture decoding apparatus which has obtained thispicture is capable of easily determining, based on the position flag,whether the reference block is located before or after the currentblock. Thus, even when the reference block has two reference motionvectors, the picture decoding apparatus is capable of easily selecting asuitable reference motion vector, and, for example, by scaling theselected reference motion vector, the picture decoding apparatus iscapable of calculating or deriving the most suitable motion vector forthe current block to be coded (or to be decoded). As a result, it ispossible to appropriately decode the current block coded at the highcompression rate.

Furthermore, it may be that in the selection step, in the case where thereference block has, as the two reference motion vectors, a forwardreference motion vector directed forward and a backward reference motionvector directed backward, the forward reference motion vector isselected from among the two reference motion vectors when the positionflag indicates that the reference block is located after the currentblock, and the backward reference motion vector is selected from amongthe two reference motion vectors when the position flag indicates thatthe reference block is located before the current block.

With this, a suitable reference motion vector can be reliably selectedaccording to the position flag.

Furthermore, it may be that the coding step includes: a comparison stepof comparing coding efficiency of the current block according to themotion vector calculated in the calculation step and coding efficiencyof the current block according to a motion vector resulting from motionestimation for the current block; a motion vector selection step ofselecting, based on a result of the comparison in the comparison step, amotion vector having high coding efficiency from among the motion vectorcalculated in the calculation step and the motion vector resulting frommotion estimation; and a block coding step of coding the current blockaccording to the motion vector selected in the motion vector selectionstep.

With this, out of the motion vector calculated in temporal direct forthe current block and the motion vector resulting from motion estimationfor the current block, a motion vector having higher coding efficiencyis selected, and according to the selected motion vector, the currentblock is coded, which allows a further increase in the compression rateor the coding efficiency.

Furthermore, in order to achieve the above object, a picture decodingmethod according to an aspect of the present invention is a picturedecoding method of decoding a coded moving picture, comprising: theselection step and the calculation step in the motion vector calculationmethod according to one of the above aspects of the present invention;and a decoding step of decoding the coded current block included in thecoded moving picture, using the motion vector calculated in thecalculation step.

With this, even when the reference block has two reference motionvectors, the current block can be decoded using the most suitable motionvector calculated for the current block, which allows appropriatedecoding of the current block coded at a high compression rate.

Furthermore, it may be that the picture decoding method furthercomprises an obtaining step of obtaining a position flag added to apicture including the current block, wherein, in the selection step, inthe case where the reference block has, as the two reference motionvectors, a forward reference motion vector directed forward and abackward reference motion vector directed backward, the forwardreference motion vector is selected from among the two reference motionvectors when the position flag indicates that the reference block islocated after the current block, and the backward reference motionvector is selected from among the two reference motion vectors when theposition flag indicates that the reference block is located before thecurrent block.

With this, since a picture including the current block has a positionflag, it is possible to easily determine, based on the position flag,whether the reference block is located before or after the currentblock. Thus, even when the reference block has two reference motionvectors, a suitable reference motion vector can be easily selectedaccording to the position flag, and, for example, by scaling theselected reference motion vector, it is possible to calculate or derivethe most suitable motion vector for the current block to be coded (or tobe decoded). As a result, it is possible to appropriately decode thecurrent block coded at the high compression rate.

It is to be noted that the present invention can be implemented not onlyas the above motion vector calculation method, picture coding method,and picture decoding method, but also as a device and an integratedcircuit which operate according to those methods, a program which causesa computer to operate according to those methods, and a recording mediumor the like in which the program is stored.

Advantageous Effects of Invention

According to the present invention, the use of new criteria forselecting the motion vector to be used in the temporal direct allows notonly derivation of the motion vector most suitable for the currentblock, but also an increase in the compression rate.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 illustrates an inter prediction coding process in temporal directand a method of calculating a motion vector.

FIG. 2 is a block diagram showing a configuration of a picture codingapparatus using a picture coding method according to Embodiment 1 of thepresent invention.

FIG. 3 shows an outline of a process flow of the picture coding methodaccording to Embodiment 1 of the present invention.

FIG. 4 shows a determination flow of an inter prediction control unit inan inter prediction mode according to Embodiment 1 of the presentinvention.

FIG. 5 shows a detailed process flow in Step S110 of FIG. 3 according toEmbodiment 1 of the present invention.

FIG. 6 shows an example of a method of deriving a motion vector (atemporal direct vector) in temporal direct according to Embodiment 1 ofthe present invention.

FIG. 7 shows another example of the method of deriving a motion vector(a temporal direct vector) in temporal direct according to Embodiment 1of the present invention.

FIG. 8 shows another example of the method of deriving a motion vector(a temporal direct vector) in temporal direct according to Embodiment 1of the present invention.

FIG. 9 shows another example of the method of deriving a motion vector(a temporal direct vector) in temporal direct according to Embodiment 1of the present invention.

FIG. 10 shows a detailed process flow of calculation of a temporaldirect vector according to Embodiment 2 of the present invention.

FIG. 11 shows an example of a method of deriving a motion vector (atemporal direct vector) in temporal direct according to Embodiment 2 ofthe present invention.

FIG. 12 shows another example of the method of deriving a motion vector(a temporal direct vector) in temporal direct according to Embodiment 2of the present invention.

FIG. 13 shows a detailed process flow of calculation of a temporaldirect vector according to Embodiment 3 of the present invention.

FIG. 14 shows a detailed process flow in Steps S110 and S120 of FIG. 3according to Embodiment 4 of the present invention.

FIG. 15 is a block diagram showing a configuration of a picture decodingapparatus using a picture decoding method according to Embodiment 5 ofthe present invention.

FIG. 16 shows an outline of a process flow of the picture decodingmethod according to Embodiment 5 of the present invention.

FIG. 17 illustrates an overall configuration of a content providingsystem for implementing content distribution services.

FIG. 18 illustrates an overall configuration of a digital broadcastingsystem.

FIG. 19 is a block diagram illustrating an example of a configuration ofa television.

FIG. 20 is a block diagram illustrating an example of a configuration ofan information reproducing/recording unit that reads and writesinformation from or on a recording medium that is an optical disk.

FIG. 21 shows an example of a configuration of a recording medium thatis an optical disk.

FIG. 22A shows an example of a cellular phone.

FIG. 22B shows an example of a configuration of the cellular phone.

FIG. 23 shows a structure of multiplexed data.

FIG. 24 schematically illustrates how each of streams is multiplexed inmultiplexed data.

FIG. 25 illustrates how a video stream is stored in a stream of PESpackets in more detail.

FIG. 26 shows a structure of TS packets and source packets in themultiplexed data.

FIG. 27 shows a data structure of a PMT.

FIG. 28 shows an internal structure of multiplexed data information.

FIG. 29 shows an internal structure of stream attribute information.

FIG. 30 shows steps for identifying video data.

FIG. 31 is a block diagram illustrating an example of a configuration ofan integrated circuit for implementing the moving picture coding methodand the moving picture decoding method according to each of Embodiments.

FIG. 32 shows a configuration for switching between driving frequencies.

FIG. 33 shows steps for identifying video data and switching betweendriving frequencies.

FIG. 34 shows an example of a look-up table in which standards of videodata are associated with the driving frequencies.

FIG. 35A shows an example of a configuration for sharing a module of asignal processing unit.

FIG. 35B shows another example of a configuration for sharing a moduleof a signal processing unit.

DESCRIPTION OF EMBODIMENTS

Embodiments of the present invention are described below with referenceto the drawings.

Embodiment 1

FIG. 2 is a block diagram showing a structure of a picture codingapparatus using a picture coding method according to Embodiment 1 of thepresent invention.

A picture coding apparatus 100 according to this embodiment includes, asshown in FIG. 2, a subtractor 101, an adder 106, an orthogonal transformunit 102, a quantization unit 103, an inverse quantization unit 104, aninverse orthogonal transform unit 105, a block memory 107, a framememory 108, an intra prediction unit 109, an inter prediction unit 110,an inter prediction control unit 111, a picture type determination unit112, a temporal direct vector calculation unit 113, a co-locatedreference direction determination unit 114, and a variable-length codingunit 115.

The subtractor 101 generates prediction error picture data bysubtracting, from an input picture sequence that is moving pictures,prediction picture data generated by the intra prediction unit 109 orthe inter prediction unit 110.

The orthogonal transform unit 102 transforms an area of the predictionerror picture data from image domain into frequency domain.

The quantization unit 103 performs a quantization process on acoefficient sequence that is the prediction error picture datatransformed into frequency domain.

The inverse quantization unit 104 performs an inverse quantizationprocess on the coefficient sequence treated with the quantizationprocess of the quantization unit 103.

The inverse orthogonal transform unit 105 transforms, from frequencydomain into image domain, the area of the coefficient sequence treatedwith the inverse quantization process.

The adder 106 generates a reconstructed picture data by adding, to theprediction picture data, the prediction error picture data that is thecoefficient sequence transformed by the inverse orthogonal transformunit 105 into image domain.

The block memory 107 stores the reconstructed picture data in units ofblocks, and the frame memory 108 stores the reconstructed picture datain units of frames.

The picture type determination unit 112 determines which one of thepicture types: I-picture, B-picture, and P-picture, is used to code apicture included in the input picture sequence, and generates picturetype information indicating the determined picture type.

The intra prediction unit 109 performs intra prediction for the currentblock (to be processed) using the reconstructed picture data stored inunits of blocks in the block memory 107, and thereby generatesprediction picture data.

The inter prediction unit 110 performs inter prediction for the currentblock using the reconstructed picture data stored in units of frames inthe frame memory 108, and thereby generates prediction picture data.

The co-located reference direction determination unit 114 determineswhich one of a block included in a picture located before the currentpicture or block (to be processed) in display time order (hereinafterreferred to as a forward reference block) and a block included in apicture located after the current picture or block (to be processed) indisplay time order (hereinafter referred to as a backward referenceblock) will be a co-located block. The co-located block will be areference block to which the current block refers. The co-locatedreference direction determination unit 114 then generates a co-locatedreference direction flag (a position flag) for each picture to add theco-located reference direction flag (the position flag) to the currentpicture. Here, the co-located block indicates a block which is includedin a picture different from a picture including the current block andwhose position in the picture is the same as that of the current block.It is to be noted that, as long as the co-located block is a blockincluded in a picture different from the picture including the currentblock, the position, in the picture, of the co-located block may bedifferent from the position of the current block.

In the case where the co-located block is a forward reference blockhaving two or more motion vectors, the temporal direct vectorcalculation unit 113 derives a motion vector of the current block intemporal direct using one of the motion vectors which is directedbackward in display time order (hereinafter referred to as a backwardreference motion vector). On the other hand, in the case where theco-located block is a backward reference block having two or more motionvectors, the temporal direct vector calculation unit 113 derives amotion vector of the current block in temporal direct using one of themotion vectors which is directed forward in display time order(hereinafter referred to as a forward reference motion vector). It is tobe noted that the forward reference motion vector and the backwardreference motion vector of the co-located block are collectivelyreferred to as a reference motion vector below. In addition, thetemporal direct vector calculation unit 113 is configured as a motionvector calculation apparatus.

Furthermore, in the case where the co-located block has only onereference motion vector, the temporal direct vector calculation unit 113derives a motion vector of the current block in temporal direct usingthe one reference motion vector of in the co-located block. For example,the temporal direct vector calculation unit 113 determines whether ornot the one reference motion vector is a forward reference motionvector, and when the one reference motion vector is a forward referencemotion vector, the temporal direct vector calculation unit 113 derives amotion vector of the current block in temporal direct using the forwardreference motion vector. On the other hand, when the one referencemotion vector is not a forward reference motion vector, the temporaldirect vector calculation unit 113 derives a motion vector of thecurrent block in temporal direct using a backward reference motionvector. Furthermore, when the co-located block has no reference motionvectors, the temporal direct vector calculation unit 113 stops thederivation of a motion vector in temporal direct, or derives a motionvector of the current block assuming that the reference motion vector is0.

The inter prediction control unit 111 determines a motion vector to beused in inter prediction. Specifically, for the current block, the interprediction control unit 111 compares the motion vector resulting frommotion estimation and the motion vector derived in temporal direct, todetermine a motion vector having higher accuracy as the motion vector tobe used in inter prediction. In addition, the inter prediction controlunit 111 generates, for each block, an inter prediction mode flagindicating whether the motion vector is to be derived by motionestimation or in temporal direct, and adds the generated interprediction mode flag to the current block.

The variable-length coding unit 115 generates a bit stream that is codedmoving pictures, by performing a variable-length coding process on thecoefficient sequence treated with the quantization process, the interprediction mode flag, the picture type information, and the co-locatedreference direction flag.

FIG. 3 shows an outline of a process flow of the picture coding methodaccording to this embodiment of the present invention.

The temporal direct vector calculation unit 113 selects one of thereference motion vectors of the co-located block and calculates a motionvector of the current block (to be processed), using the selected one ofthe reference motion vectors (Step S110). It is to be noted that Step110 corresponds to the motion vector calculation method according to animplementation of the present invention. Specifically, when theco-located block has two reference motion vectors, the temporal directvector calculation unit 113 selects one of the two reference motionvectors based on whether the co-located block is located, in displaytime order, before the current block (to be processed) (that is, theco-located block is the forward reference block) or after the currentblock (to be processed) (that is, the co-located block is the backwardreference block), and when the co-located block has only one referencemotion vector, the temporal direct vector calculation unit 113 selectsthe one reference motion vector. For example, in the case where theco-located block is the forward reference block having two or morereference motion vectors, the temporal direct vector calculation unit113 derives a motion vector of the current block in temporal directusing the backward reference motion vector. On the other hand, in thecase where the co-located block is the backward reference block havingtwo or more reference motion vectors, the temporal direct vectorcalculation unit 113 derives a motion vector of the current block intemporal direct using the forward reference motion vector. It is to benoted that the motion vector derived in temporal direct is referred toas a temporal direct vector.

Here, the co-located block can be determined as follows. That is, theco-located reference direction determination unit 114 determines whichone of the forward reference block and the backward reference block willbe the co-located block, when the motion vector of the current block isderived in temporal direct. Furthermore, for example, the co-locatedreference direction determination unit 114 may generate, for eachpicture, the co-located reference direction flag indicating whether theco-located block is the forward reference block or the backwardreference block, and add the generated co-located reference directionflag to the picture. It is to be noted that the addition of theco-located reference direction flag is not limited to the addition inunits of pictures and may be addition in units of slices included in thepictures.

Next, the picture coding apparatus 100 codes the current block (to beprocessed) using the motion vector calculated by the temporal directvector calculation unit 113 (Step S120). Specifically, the interprediction unit 110 generates prediction picture data using the motionvector calculated by the temporal direct vector calculation unit 113(Step S120). For example, the inter prediction control unit 111 comparesthe motion vector resulting from motion estimation and the motion vectorderived in temporal direct, to select the motion vector having higheraccuracy. Furthermore, using the selected motion vector, the interprediction unit 110 performs inter prediction for the current block andthereby generates prediction picture data. Here, the inter predictioncontrol unit 111 determines, as an inter prediction mode, whether toderive a motion vector by motion estimation or in temporal direct, togenerate, for each block, an inter prediction mode flag indicating thatinter prediction mode, and adds the generated inter prediction mode flagto the current block. It is to be noted that the motion vector resultingfrom motion estimation is referred to as a motion estimation resultvector. Using the prediction picture data generated as above, thecurrent block (to be processed) is coded.

FIG. 4 shows a determination flow of the inter prediction control unit111 in the inter prediction mode.

The inter prediction control unit 111 calculates SADinter that isinformation on a difference between the original picture (the currentblock) and the prediction picture data generated using the motion vectorresulting from motion estimation (Step S131). Here, a sum of absolutedifference (SAD) represents a sum of absolute values of differences inrespective pixels between the original picture and the predictionpicture data. Furthermore, SADinter represents SAD between the originalpicture and the prediction picture data generated using the motionvector resulted from motion estimation. The inter prediction controlunit 111 calculates SADdirect that is information on a differencebetween the original picture and the prediction picture data generatedusing the motion vector derived in temporal direct (Step S132). Here,SADdirect represents SAD between the original picture and the predictionpicture data generated using the motion vector derived in temporaldirect.

Next, the inter prediction control unit 111 compares SADinter andSADdirect (Step S133). Here, when SADinter is smaller, that is, themotion estimation result vector has higher accuracy (Yes in Step S133),the inter prediction control unit 111 determines the use of a motionestimation mode as the inter prediction mode (Step S134). On the otherhand, when SADdirect is smaller, that is, the temporal direct vector hashigher accuracy (No in Step S133), the inter prediction control unit 111determines the use of a temporal direct mode as the inter predictionmode (Step S135).

At the end, the inter prediction control unit 111 generates, for eachblock, the inter prediction mode flag indicating the determined interprediction mode, and adds the generated inter prediction mode flag tothe current block.

Although whether to use the temporal direct mode is determined using SADin this embodiment, it is possible to use, for example, SSD that is asum of square differences in respective pixels between the originalpicture and the prediction picture data.

FIG. 5 shows a detailed process flow in Step S110 of FIG. 3. Thefollowing describes about FIG. 5. It is to be noted that FIG. 5 shows anexample of the motion vector calculation unit according to the presentinvention.

First, the temporal direct vector calculation unit 113 determineswhether the co-located block has two or more reference motion vectors,that is, has at least the forward reference motion vector (mvL0) and thebackward reference motion vector (mvL1) (Step S121). When it isdetermined in Step S121 that the co-located block has two or morereference motion vectors (Yes in Step S121), the temporal direct vectorcalculation unit 113 determines whether or not the co-located block islocated after the current block, that is, whether or not the co-locatedblock is the backward reference block (Step S122). When the co-locatedblock is the backward reference block (Yes in Step S122), the temporaldirect vector calculation unit 113 derives a motion vector of thecurrent block in temporal direct using the forward reference motionvector mvL0 of the co-located block (Step S123). On the other hand, whenthe co-located block is the forward reference block (No in Step S122),the temporal direct vector calculation unit 113 derives a motion vectorof the current block in temporal direct using the backward referencemotion vector mvL1 of the co-located block (Step S124).

Furthermore, when it is determined in Step S121 that the co-locatedblock has only one of the forward reference motion vector mvL0 and thebackward reference motion vector mvL1 (No in Step S121), the temporaldirect vector calculation unit 113 determines whether or not theco-located block has the forward reference motion vector mvL0 (StepS125). When it is determined in Step S125 that the co-located block hasthe forward reference motion vector mvL0 (Yes in Step S125), thetemporal direct vector calculation unit 113 derives a motion vector ofthe current block in temporal direct using the forward reference motionvector mvL0 of the co-located block (Step S126). On the other hand, whenit is determined in Step S125 that the co-located block has no forwardreference motion vector mvL0 (No in Step S125), the temporal directvector calculation unit 113 determines whether or not the co-locatedblock has the backward reference motion vector mvL1 (Step S127).

Here, when it is determined in Step S127 that the co-located block hasthe backward reference motion vector mvL1, the temporal direct vectorcalculation unit 113 derives a motion vector of the current block intemporal direct using the backward reference motion vector mvL1 of theco-located block (Step S128). On the other hand, when it is determinedin Step S127 that the co-located block has no backward reference motionvector mvL1 (No in Step S127), the temporal direct vector calculationunit 113 stops the derivation of a motion vector of the current block intemporal direct. Alternatively, the temporal direct vector calculationunit 113 derives a motion vector of the current block in temporal directassuming that the reference motion vector of the co-located block is 0(Step S129).

In the process flow of FIG. 5, it is determined in S125 whether or notthe co-located block has the forward reference motion vector mvL0, andit is determined in Step S127 whether or not the co-located block hasthe backward reference motion vector mvL1, but the present invention isnot limited to this flow. For example, whether or not the co-locatedblock has the forward reference motion vector mvL0 may be determinedafter determination on whether or not the co-located block has thebackward reference motion vector mvL1. Furthermore, as described above,(i) when the co-located block has two reference motion vectors, thetemporal direct vector calculation unit 113 selects one of the tworeference motion vectors which is to be used in temporal direct,according to the position of the co-located block, (ii) when theco-located block has only one reference motion vector, the temporaldirect vector calculation unit 113 selects the one reference motionvector as the motion vector to be used in temporal direct, and (iii)when the co-located block has no reference motion vector, the temporaldirect vector calculation unit 113 stops the derivation of a motionvector in temporal direct. Thus, it is sufficient that, according toeach of the above cases, the temporal direct vector calculation unit 113performs a process which corresponds to the case, and the determinations(such as Steps S121, S122, S125, and S127) on which of the cases appliesto the status of the co-located block may be performed in any order.

Next, a method of deriving a motion vector in temporal direct isdescribed in detail.

FIG. 6 shows an example of a method of deriving a motion vector (atemporal direct vector) in temporal direct according to this embodiment.The co-located block is the backward reference block and has the forwardreference motion vector mvL0 and the backward reference motion vectormvL1. In this case, using the forward reference motion vector mvL0, thetemporal direct vector calculation unit 113 derives a motion vector(TMV) of the current block by the following calculation expression(Expression 1):TMV=mvL0×(B2−B0)/(B4−B0)  (Expression 1)

Here, (B2−B0) represents information on a time difference in displaytime between a picture B2 and a picture B0, and (B4−B0) representsinformation on a time difference in display time between a picture B4and the picture B0.

FIG. 7 shows another example of the method of deriving a motion vector(a temporal direct vector) in temporal direct according to thisembodiment. The co-located block is the backward reference block and hasonly the backward reference motion vector mvL1. In this case, using thebackward reference motion vector mvL1, the temporal direct vectorcalculation unit 113 derives a motion vector (TMV) of the current blockby the following calculation expression (Expression 2):TMV=mvL1×(B2−B0)/(B4−B8)  (Expression 2)

FIG. 8 shows another example of the method of deriving a motion vector(a temporal direct vector) in temporal direct according to thisembodiment. The co-located block is the forward reference block and hasthe forward reference motion vector mvL0 and the backward referencemotion vector mvL1. In this case, using the backward reference motionvector mvL1, the temporal direct vector calculation unit 113 derives amotion vector (TMV) of the current block by the following calculationexpression (Expression 3):TMV=mvL1×(B6−B8)/(B4−B8)  (Expression 3)

FIG. 9 shows another example of the method of deriving a motion vector(a temporal direct vector) in temporal direct according to thisembodiment. The co-located block is the forward reference block and hasonly the forward reference motion vector mvL0. In this case, using theforward reference motion vector mvL0, the temporal direct vectorcalculation unit 113 derives a motion vector (TMV) of the current blockby the following calculation expression (Expression 4):TMV=mvL0×(B6−B8)/(B4−B0)  (Expression 4)

Thus, according to this embodiment, when the co-located block has two ormore reference motion vectors, the motion vector most suitable for thecurrent block can be derived, which allows an increase in thecompression rate. Especially, when the co-located block is the forwardreference block, the use of the backward reference motion vector allowsa reduction in the prediction error. In this case, the backwardreference motion vector is a motion vector directed from a pictureincluding the co-located block to a picture including the current block.Accordingly, the motion vector derived from the backward referencemotion vector has a higher probability of approximating the mostsuitable motion vector, which reduces the prediction error. On the otherhand, the forward reference motion vector is a motion vector in adirection opposite to the direction from the picture including theco-located block to the picture including the current block.Accordingly, the motion vector derived from the forward reference motionvector has a lower probability of approximating the most suitable motionvector, which increases the prediction error. Likewise, also in the casewhere the co-located block is the backward reference block, the motionvector derived from the forward reference motion vector has a higherprobability of approximating the most suitable motion vector, with theresult that the use of the forward reference motion vector allows areduction in the prediction error.

Embodiment 2

In this embodiment, when the co-located block has two or more referencemotion vectors, it is determined to use, in temporal direct, a referencemotion vector which refers to a picture temporally close to the pictureincluding the co-located block, that is, a reference motion vectorhaving a short temporal distance to the picture including the co-locatedblock. In this regard, this embodiment is different in structure fromother embodiments. Here, the temporal distance is determined accordingto the number of pictures in display time order between the pictureincluding the co-located block and the reference motion picture to whichthe co-located block refers. Specifically, this embodiment is differentfrom Embodiment 1 in the process in Step S110 in the process flow shownin FIG. 3.

FIG. 10 shows a detailed process flow of calculation of a temporaldirect vector according to this embodiment. The following describesabout FIG. 10.

First, the temporal direct vector calculation unit 113 determineswhether the co-located block has two or more reference motion vectors,that is, has at least the forward reference motion vector mvL0 and thebackward reference motion vector mvL1 (Step S121). When it is determinedin Step S121 that the co-located block has two or more reference motionvectors (Yes in Step S121), the temporal direct vector calculation unit113 determines whether or not a reference picture to which the forwardreference motion vector mvL0 refers is temporally closer to the pictureincluding the co-located block (the co-located picture) than a referencepicture to which the backward reference motion vector mvL1 refers is(Step S122 a). In short, the temporal direct vector calculation unit 113determines which one of the forward reference motion vector mvL0 and thebackward reference motion vector mvL1 has a shorter temporal distance.When it is determined in Step S122 a that the reference picture to whichthe forward reference motion vector mvL0 refers is temporally closer tothe picture including the co-located block (Yes in Step S122 a), thetemporal direct vector calculation unit 113 derives a motion vector ofthe current block in temporal direct using the forward reference motionvector mvL0 of the co-located block (Step S123). On the other hand, whenit is determined in Step S122 a that the reference picture to which thebackward reference motion vector mvL1 refers is temporally closer to thepicture including the co-located block (No in Step S122 a), the temporaldirect vector calculation unit 113 derives a motion vector of thecurrent block in temporal direct using the backward reference motionvector mvL1 of the co-located block (Step S124).

Furthermore, when it is determined in Step S121 that the co-locatedblock has only one of the forward reference motion vector mvL0 and thebackward reference motion vector mvL1 (No in Step S121), the temporaldirect vector calculation unit 113 determines whether or not theco-located block has the forward reference motion vector (Step S125).When it is determined in Step S125 that the co-located block has theforward reference motion vector (Yes in Step S125), the temporal directvector calculation unit 113 derives a motion vector of the current blockin temporal direct using the forward reference motion vector mvL0 of theco-located block (Step S126). On the other hand, when it is determinedin Step S125 that the co-located block has no forward reference motionvector mvL0 (No in Step S125), the temporal direct vector calculationunit 113 determines whether or not the co-located block has the backwardreference motion vector mvL1 (Step S127).

Here, when it is determined in Step S127 that the co-located block hasthe backward reference motion vector mvL1 (Yes in Step S127), thetemporal direct vector calculation unit 113 derives a motion vector ofthe current block in temporal direct using the backward reference motionvector mvL1 of the co-located block (Step S128). On the other hand, whenit is determined in Step S127 that the co-located block has no backwardreference motion vector mvL1 (No in Step S127), the temporal directvector calculation unit 113 stops the derivation of a motion vector ofthe current block in temporal direct. Alternatively, the temporal directvector calculation unit 113 derives a motion vector of the current blockin temporal direct assuming that the reference motion vector of theco-located block is 0 (Step S129). That is, the motion vector of thecurrent block is 0.

Next, a method of deriving a motion vector in temporal direct accordingto this embodiment is described in detail.

FIG. 11 shows an example of the method of deriving a motion vector (atemporal direct vector) in temporal direct according to this embodiment.The co-located block has the forward reference motion vector mvL0 andthe backward reference motion vector mvL1. The reference picture towhich the forward reference motion vector mvL0 refers is temporallycloser to the picture including the co-located block than the referencepicture to which the backward reference motion vector mvL1 refers is. Inthis case, using the forward reference motion vector mvL0, the temporaldirect vector calculation unit 113 derives a motion vector (TMV) of thecurrent block by the following calculation expression (Expression 5):TMV=mvL0×(B2−B0)/(B4−B0)  (Expression 5)

FIG. 12 shows another example of the method of deriving a motion vector(a temporal direct vector) in temporal direct according to thisembodiment. The co-located block has the forward reference motion vectormvL0 and the backward reference motion vector mvL1. The referencepicture to which the backward reference motion vector mvL1 refers istemporally closer to the picture including the co-located block than thereference picture to which the forward reference motion vector mvL0refers is. In this case, using the backward reference motion vectormvL1, the temporal direct vector calculation unit 113 derives a motionvector (TMV) of the current block by the following calculationexpression (Expression 6):TMV=mvL1×(B2−B0)/(B4−B8)  (Expression 6)

Thus, according to this embodiment, when the co-located block has two ormore reference motion vectors, the motion vector most suitable for thecurrent block can be derived, which allows an increase in thecompression rate. Specifically, by using the reference motion vectorwhich refers to a picture temporally close to the picture including theco-located block, the motion vector derived from that reference motionvector has a higher probability of approximating the most suitablemotion vector, which allows a reduction in the prediction error.

It is to be noted that this embodiment can be combined with otherembodiments. For example, Step S122 shown in FIG. 5 according toEmbodiment 1 may be combined with Step 122 a shown in FIG. 10 accordingto this embodiment. In this case, Step 122 a shown in FIG. 10 accordingto this embodiment is given priority over Step S122 shown in FIG. 5according to Embodiment 1. Specifically, first, a temporal distancebetween two or more reference motion vectors is determined to select amotion vector having a short temporal distance. Here, when the two ormore reference motion vectors have an equal temporal distance, areference motion vector is selected based on whether the co-locatedblock is the forward reference block or the backward reference block.Since the impact on the motion vector is larger in Step 122 a accordingto this embodiment, prioritizing the temporal distance of the referencemotion vector allows a selection of a more suitable reference motionvector.

Embodiment 3

In this embodiment, when the co-located block has two or more referencemotion vectors, it is determined to use, in temporal direct, a referencemotion vector having a smaller magnitude. In this regard, thisembodiment is different in structure from other embodiments. Here, themagnitude of the motion vector means the absolute value of the motionvector. Specifically, this embodiment is different from Embodiment 1 inthe process in Step S110 in the process flow shown in FIG. 3.

FIG. 13 shows a detailed process flow of calculation of a temporaldirect vector according to this embodiment. The following describesabout FIG. 13.

First, the temporal direct vector calculation unit 113 determineswhether or not the co-located block has two or more reference motionvectors, that is, has at least the forward reference motion vector mvL0and the backward reference motion vector mvL1 (Step S121). When it isdetermined in Step S121 that the co-located block has two or morereference motion vectors (Yes in Step S121), the temporal direct vectorcalculation unit 113 determines whether or not the magnitude of theforward reference motion vector mvL0 is smaller than the magnitude ofthe backward reference motion vector mvL1 (Step S122 b). When it isdetermined in Step S122 b that the magnitude of the forward referencemotion vector mvL0 is smaller (Yes in Step S122 b), the temporal directvector calculation unit 113 derives a motion vector of the current blockin temporal direct using the forward reference motion vector mvL0 of theco-located block (Step S123). On the other hand, when it is determinedin Step S122 b that the magnitude of the backward reference motionvector mvL1 is smaller (No in Step S122 b), the temporal direct vectorcalculation unit 113 derives a motion vector of the current block intemporal direct using the backward reference motion vector mvL1 of theco-located block (Step S124).

Furthermore, when it is determined in Step S121 that the co-locatedblock has only one of the forward reference motion vector mvL0 and thebackward reference motion vector mvL1 (No in Step S121), the temporaldirect vector calculation unit 113 determines whether or not theco-located block has the forward reference motion vector (Step S125).When it is determined in Step S125 that the co-located block has theforward reference motion vector (Yes in Step S125), the temporal directvector calculation unit 113 derives a motion vector of the current blockin temporal direct using the forward reference motion vector mvL0 of theco-located block (Step S126). On the other hand, when it is determinedin Step S125 that the co-located block has no forward reference motionvector mvL0 (No in Step S125), the temporal direct vector calculationunit 113 determines whether or not the co-located block has the backwardreference motion vector mvL1 (Step S127).

Here, when it is determined in Step S127 that the co-located block hasthe backward reference motion vector mvL1 (Yes in Step S127), thetemporal direct vector calculation unit 113 derives a motion vector ofthe current block in temporal direct using the backward reference motionvector mvL1 of the co-located block (Step S128). On the other hand, whenit is determined in Step S127 that the co-located block has no backwardreference motion vector mvL1 (No in Step S127), the temporal directvector calculation unit 113 stops the derivation of a motion vector ofthe current block in temporal direct. Alternatively, the temporal directvector calculation unit 113 derives a motion vector of the current blockin temporal direct assuming that the reference motion vector of theco-located block is 0 (Step S129). That is, the motion vector of thecurrent block is 0.

Thus, according to this embodiment, when the co-located block has two ormore reference motion vectors, the motion vector most suitable for thecurrent block can be derived, which allows an increase in thecompression rate.

It is to be noted that this embodiment can be combined with otherembodiments. For example, Step S122 shown in FIG. 5 according toEmbodiment 1 may be combined with Step 122 b shown in FIG. 13 accordingto this embodiment. In this case, Step 122 b shown in FIG. 13 accordingto this embodiment is given priority over Step S122 shown in FIG. 5according to Embodiment 1. Specifically, first, the magnitudes of two ormore reference motion vectors are determined to select a motion vectorhaving a small magnitude. Here, when the two or more reference motionvectors have an equal magnitude, a reference motion vector is selectedbased on whether the co-located block is the forward reference block orthe backward reference block. Since the impact on the motion vector islarger in Step 122 b according to this embodiment, prioritizing themagnitude of the reference motion vector allows a selection of a moresuitable reference motion vector.

Embodiment 4

In this embodiment, in the case where the co-located block indicated bythe co-locate reference direction flag has no reference motion vectorand the motion vector of the current block cannot be derived in temporaldirect, that is, in the case where the motion vector is 0, theco-located block is changed, and a motion vector of the current block iscalculated to perform inter prediction.

First, in Step S110 of FIG. 3, the co-located reference directiondetermination unit 114 determines which one of the forward referenceblock and the backward reference block will be the co-located block.When it is determined that the forward reference block will be theco-located block, the co-located reference direction determination unit114 sets a co-located backward reference priority flag (the co-locatedreference direction flag) at 0. On the other hand, when it is determinedthat the backward reference block will be the co-located block, theco-located reference direction determination unit 114 sets theco-located backward reference priority flag at 1. Here, the co-locatedreference direction determination unit 114 generates the co-locatedbackward reference priority flag for each picture and then adds thegenerated co-located backward reference priority flag to the picture tobe coded.

Since the co-located backward reference priority flag is generated inunits of pictures, there may be a case where the co-located blockcorresponding to a certain block included in the current picture hasneither the forward reference motion vector nor the backward referencemotion vector. In this case, the temporal direct vector calculation unit113 changes the co-located block to derive a more suitable motionvector.

FIG. 14 shows a detailed process flow in Steps S110 and S120 of FIG. 3according to this embodiment. The following describes about FIG. 14.

The temporal direct vector calculation unit 113 determines whether ornot the co-located backward reference priority flag is 1, that is,whether or not the backward reference block is to be prioritized (StepS141). When the co-located backward reference priority flag is 1 (Yes inStep S141), the temporal direct vector calculation unit 113 attempts toderive a motion vector of the current block in temporal direct using theco-located block which is the backward reference block (Step S142). Thetemporal direct vector calculation unit 113 determines whether or notthe motion vector of the current block has been derived in Step S142,that is, whether or not the motion vector is 0 (Step S143). When themotion vector has not been derived (Yes in Step S143), the temporaldirect vector calculation unit 113 derives a motion vector of thecurrent block in temporal direct using the co-located block which is theforward reference block (Step S144).

On the other hand, when the co-located backward reference priority flagis 0 (No in Step S141), the temporal direct vector calculation unit 113attempts to derive a motion vector of the current block in temporaldirect using the co-located block which is the forward reference block(Step S146). The temporal direct vector calculation unit 113 determineswhether or not the motion vector of the current block has been derivedin Step S146, that is, whether or not the motion vector is 0 (StepS147). When the motion vector has not been derived (Yes in Step S147),the temporal direct vector calculation unit 113 derives a motion vectorof the current block in temporal direct using the co-located block whichis the backward reference block (Step 148).

At the end, the inter prediction control unit 111 compares the motionvector resulting from motion estimation and the motion vector derived intemporal direct, to determine a motion vector having higher accuracy asthe motion vector of the current block. In other words, the interprediction control unit 111 determines the inter prediction mode.Furthermore, the inter prediction unit 110 performs inter predictionusing the determined motion vector and thereby generates the predictionpicture data (Step S145). At this time, the inter prediction controlunit 111 generates, for each block, the inter prediction mode flagindicating the inter prediction mode, and adds the generated interprediction mode flag to the current block.

Thus, according to this embodiment, in the case where the co-locatedblock indicated by the co-located backward reference priority flag hasno reference motion vector, a block in other picture is determined asthe co-located block so that a motion vector can be derived. Forexample, in the case where the backward reference block is theco-located block and the co-located block has no reference motionvector, the forward reference block is determined as the co-locatedblock so that a motion vector can be derived. This allows derivation ofa motion vector having higher accuracy.

It is to be noted that this embodiment can be combined with otherembodiments. For example, Step S129 of FIG. 5 according to Embodiment 1is replaced by a step of determining whether or not the co-located blockis the backward reference block. Specifically, in the case where theco-located block is the backward reference block, the process in StepS144 of FIG. 14 according to this embodiment is performed, while, in thecase where the co-located block is the forward reference block, theprocess in Step S148 of FIG. 14 according to this embodiment isperformed. This allows derivation of a more suitable motion vector.Likewise, Step S129 of FIG. 10 according to Embodiment 2 and Step S129of FIG. 13 according to Embodiment 3 may be replaced by a step ofdetermining whether or not the co-located block is the backwardreference block. Specifically, in the case where the co-located block isthe backward reference block, the process in Step S144 of FIG. 14according to this embodiment is performed, while, in the case where theco-located block is the forward reference block, the process in StepS148 of FIG. 14 according to this embodiment is performed. This allowsderivation of a more suitable motion vector.

In addition, when a newly selected co-located block has two or morereference motion vectors, a motion vector is derived in the methoddescribed in Embodiments 1, 2, and 3. This allows derivation of a motionvector having higher accuracy.

Embodiment 5

FIG. 15 is a block diagram showing a structure of a picture decodingapparatus using a picture decoding method according to Embodiment 5 ofthe present invention.

In this embodiment, a block included in a picture located, in displaytime order, before a current picture or block to be decoded (to beprocessed) is referred to as a forward reference block. In thisembodiment, a block included in a picture located, in display timeorder, after the current picture or block to be decoded (to beprocessed) is referred to as a backward reference block.

A picture decoding apparatus 200 according to this embodiment includes,as shown in FIG. 15, a variable-length decoding unit 215, an inversequantization unit 204, an inverse orthogonal transform unit 205, a blockmemory 207, a frame memory 208, an intra prediction unit 209, an interprediction unit 210, an inter prediction control unit 211, a temporaldirect vector calculation unit 213, and an adder 206.

The variable-length decoding unit 215 performs a variable-lengthdecoding process on an input bit stream to generate picture typeinformation, inter prediction mode flag, co-located reference directionflag, and bit streams treated with the variable-length decoding process(coefficient sequences treated with the quantization process).

The inverse quantization unit 204 performs an inverse quantizationprocess on the bit streams treated with the variable-length decodingprocess. The inverse orthogonal transform unit 205 transforms, fromfrequency domain into image domain, the bit streams treated with theinverse quantization process, and thereby generates prediction errorpicture data.

The adder 206 adds the prediction error picture data to the predictionpicture data generated by the intra prediction unit 209 or the interprediction unit 210, and thereby generates a decoded picture sequencethat is reconstructed picture data.

The block memory 207 stores the reconstructed picture data in units ofblocks, and the frame memory 208 stores the reconstructed picture datain units of frames.

The intra prediction unit 209 performs intra prediction using thereconstructed picture data stored in units of blocks in the block memory207, and thereby generates prediction error picture data for the currentblock (to be processed). The inter prediction unit 210 performs interprediction using the reconstructed picture data stored in units offrames in the frame memory 208, and thereby generates prediction errorpicture data for the current block.

The inter prediction control unit 211 controls the method of deriving amotion vector in the inter prediction, according to the inter predictionmode flag. In the case where the inter prediction mode flag indicatesthat a motion vector is to be derived in temporal direct (the temporaldirect mode), the inter prediction control unit 211 instructs the interprediction unit 210 to perform, using a motion vector (temporal directvector) derived in temporal direct, inter prediction according to thatmotion vector.

In the case of deriving a motion vector in temporal direct, the temporaldirect vector calculation unit 213 determines a co-located block usingthe co-located reference direction flag and derives a motion vector intemporal direct. The temporal direct vector calculation unit 213determines the backward reference block as the co-located block when theco-located reference direction flag indicates that the co-located blockis the backward reference block. On the other hand, when the co-locatedreference direction flag indicates that the co-located block is theforward reference block, the temporal direct vector calculation unit 213determines the forward reference block as the co-located block. In thecase where the co-located block has two or more reference motionvectors, the temporal direct vector calculation unit 213 selects areference motion vector to be used in temporal direct, based on whetherthe co-located block is the forward reference block or the backwardreference block.

For example, in the case where the co-located block is the backwardreference block, the temporal direct vector calculation unit 213 usesthe forward reference motion vector among the two or more referencemotion vectors of the co-located block. On the other hand, in the casewhere the co-located block is the forward reference block, the temporaldirect vector calculation unit 213 uses the backward reference motionvector among the two or more reference motion vectors of the co-locatedblock. In the case where the co-located block has only one of theforward reference motion vector and the backward reference motionvector, the temporal direct vector calculation unit 213 first searchesfor the forward reference motion vector, and when the co-located blockhas the forward reference motion vector, the temporal direct vectorcalculation unit 213 derives a motion vector of the current block intemporal direct using the forward reference motion vector. On the otherhand, when the co-located block has no forward reference motion vector,the temporal direct vector calculation unit 213 derives a motion vectorof the current block in temporal direct using the backward referencemotion vector.

FIG. 16 shows an outline of a process flow of the picture decodingmethod according to this embodiment.

The temporal direct vector calculation unit 213 selects one of thereference motion vectors of the co-located block and calculates a motionvector of the current block (to be processed), using the selected one ofthe reference motion vectors (Step S210). It is to be noted that Step210 corresponds to the motion vector calculation method according to animplementation of the present invention. Specifically, when theco-located block has two reference motion vectors, the temporal directvector calculation unit 213 selects one of the two reference motionvectors based on whether the co-located block is located, in displaytime order, before the current block (to be processed) (that is, theco-located block is the forward reference block) or after the currentblock (to be processed) (that is, the co-located block is the backwardreference block), and when the co-located block has only one referencemotion vector, the temporal direct vector calculation unit 213 selectsthe one reference motion vector. For example, when the co-located blockhas two or more reference motion vectors, the temporal direct vectorcalculation unit 213 determines based on the co-located referencedirection flag which one of the reference motion vectors is to be used.When the co-located reference direction flag indicates the backwardreference block as the co-located block, that is, when the co-locatedblock is the backward reference block, the temporal direct vectorcalculation unit 213 determines to use the forward reference motionvector among the two or more reference motion vectors. On the otherhand, when the co-located reference direction flag indicates the forwardreference block as the co-located block, that is, when the co-locatedblock is the forward reference block, the temporal direct vectorcalculation unit 213 determines to use the backward reference motionvector among the two or more reference motion vectors. Subsequently, thetemporal direct vector calculation unit 213 derives the motion vector ofthe current block in temporal direct using the determined referencemotion vector (the forward reference motion vector or the backwardreference motion vector) (Step S220).

Here, the co-located block can be determined as follows. That is, thevariable-length decoding unit 215 decodes the co-located referencedirection flag in units of picture. At this time, the temporal directvector calculation unit 213 determines based on the co-located referencedirection flag whether the forward reference block will be theco-located block or the backward reference block will be the co-locatedblock.

Next, the picture decoding apparatus 200 decodes the current coded block(to be processed) included in the coded moving picture, using the motionvector calculated by the temporal direct vector calculation unit 213(Step S220). Specifically, the inter prediction unit 210 generatesprediction picture data, using the motion vector calculated by thetemporal direct vector calculation unit 213. For example, in the casewhere the inter prediction mode flag decoded in units of blocksindicates decoding in temporal direct (the temporal direct mode), theinter prediction control unit 211 may instruct the inter prediction unit210 to perform inter prediction in temporal direct. As a result, theinter prediction unit 210 performs the inter prediction for the currentblock using the motion vector derived in temporal direct, and therebygenerates prediction picture data. Using the prediction picture datathus generated, the current block is decoded.

In this embodiment, when the reference block (the forward referenceblock or the backward reference block) has two or more reference motionvectors, it is determined based on the co-located reference directionflag which one of the reference motion vectors is to be used. However,the present invention is not limited to the determination based on theco-located reference direction flag. For example, when the co-locatedblock has two or more reference motion vectors, it may be that atemporal distance of each of the reference motion vectors is calculated,and a reference motion vector having a short temporal distance isselected. Here, the temporal distance is calculated based on the numberof pictures in display time between the reference picture including thereference block and the picture to which the reference picture refers.Furthermore, it may be that, first, the temporal distances of the two ormore reference motion vectors are calculated, and when those temporaldistances are different, the reference motion vector having a shorttemporal distance is selected, and when those temporal distances areequal, a reference motion vector is selected based on the co-locatedreference direction flag. Since the impact of the temporal distance islarger than the impact of the position of the reference block inderiving a suitable motion vector of the current block, determination byprioritizing the temporal distance allows derivation of a more suitablemotion vector.

Furthermore, it may be that, for example, when the co-located block hastwo or more reference motion vectors, the magnitudes of the referencemotion vectors are calculated and a reference motion vector having asmall magnitude is selected. Here, the magnitude of the reference motionvector means the absolute value of the motion vector. Furthermore, itmay be that, first, the magnitudes of the two or more reference motionvectors are calculated, and when those magnitudes are different, thereference motion vector having a small magnitude is selected, and whenthose magnitudes are equal, a reference motion vector is selected basedon the co-located reference direction flag. Since the impact of themagnitude of the reference motion vector is larger than the impact ofthe position of the reference block in deriving a suitable motion vectorof the current block, determination by prioritizing the magnitude of thereference motion vector allows derivation of a more suitable motionvector.

Furthermore, when the co-located block has no reference motion vector,it is also possible to derive a more suitable motion vector for thecurrent block by determining a block of a new reference picture as theco-located block. For example, when the reference picture including theco-located block is located after the current picture in display order,the co-located block included in the reference picture located beforethe current picture in display order is selected. When the referencepicture including the co-located block is located before the currentpicture in display order, the co-located block included in the referencepicture located after the current picture in display order is selected.As above, when the co-located block has no reference motion vector,selecting, as the co-located block, a block included in a new referencepicture, makes it possible to derive a motion vector having higheraccuracy for the current block. When the newly selected co-located blockhas two or more reference motion vectors, it becomes possible to derivea motion vector having higher accuracy by selecting a reference motionvector based on whether the co-located block is the forward referenceblock or the backward reference block, or based on the temporal distanceof the reference motion vector of the co-located block, or the magnitudeof the reference motion vector of the co-located block, as describedabove.

Thus, according to this embodiment, when the co-located block has two ormore reference motion vectors, the motion vector most suitable for thecurrent block can be selected, which allows a bit stream compressed athigh efficiency to be appropriately decoded.

Embodiment 6

The processing described in each of Embodiments can be simplyimplemented in an independent computer system, by recording, in arecording medium, a program for implementing the configurations of themoving picture coding method (the picture coding method) and the movingpicture decoding method (the picture decoding method) described in eachof Embodiments. The recording media may be any recording media as longas the program can be recorded, such as a magnetic disk, an opticaldisk, a magnetic optical disk, an IC card, and a semiconductor memory.

Hereinafter, the applications to the moving picture coding method (thepicture coding method) and the moving picture decoding method (thepicture decoding method) described in each of Embodiments and systemsusing them will be described. This system is characterized by includinga picture coding and decoding apparatus composed of the picture codingapparatus using the picture coding method and the picture decodingapparatus using the picture decoding method. The other structure of thesystem can be appropriately changed depending on situations.

FIG. 17 illustrates an overall configuration of a content providingsystem ex100 for implementing content distribution services. The areafor providing communication services is divided into cells of desiredsize, and base stations ex106, ex107, ex108, ex109, and ex110 which arefixed wireless stations are placed in each of the cells.

The content providing system ex100 is connected to devices, such as acomputer ex111, a personal digital assistant (PDA) ex112, a cameraex113, a cellular phone ex114 and a game machine ex115, via the Internetex101, an Internet service provider ex102, a telephone network ex104, aswell as the base stations ex106 to ex110, respectively.

However, the configuration of the content providing system ex100 is notlimited to the configuration shown in FIG. 17, and a combination inwhich any of the elements are connected is acceptable. In addition, eachdevice may be directly connected to the telephone network ex104, ratherthan via the base stations ex106 to ex110 which are the fixed wirelessstations. Furthermore, the devices may be interconnected to each othervia a short distance wireless communication and others.

The camera ex113, such as a digital video camera, is capable ofcapturing video. A camera ex116, such as a digital video camera, iscapable of capturing both still images and video. Furthermore, thecellular phone ex114 may be the one that meets any of the standards suchas Global System for Mobile Communications (GSM), Code Division MultipleAccess (CDMA), Wideband-Code Division Multiple Access (W-CDMA), LongTerm Evolution (LTE), and High Speed Packet Access (HSPA).Alternatively, the cellular phone ex114 may be a Personal HandyphoneSystem (PHS).

In the content providing system ex100, a streaming server ex103 isconnected to the camera ex113 and others via the telephone network ex104and the base station ex109, which enables distribution of images of alive show and others. In such a distribution, a content (for example,video of a music live show) captured by the user using the camera ex113is coded as described above in each of Embodiments (that is, the systemfunctions as the picture coding apparatus according to an implementationof the present invention), and the coded content is transmitted to thestreaming server ex103. On the other hand, the streaming server ex103carries out stream distribution of the transmitted content data to theclients upon their requests. The clients include the computer ex111, thePDA ex112, the camera ex113, the cellular phone ex114, and the gamemachine ex115 that are capable of decoding the above-mentioned codeddata. Each of the devices that have received the distributed datadecodes and reproduces the coded data (that is, the system functions asthe picture decoding apparatus according to the implementation of thepresent invention).

The captured data may be coded by the camera ex113 or the streamingserver ex103 that transmits the data, or the coding processes may beshared between the camera ex113 and the streaming server ex103.Similarly, the distributed data may be decoded by the clients or thestreaming server ex103, or the decoding processes may be shared betweenthe clients and the streaming server ex103. Furthermore, the data of thestill images and video captured by not only the camera ex113 but alsothe camera ex116 may be transmitted to the streaming server ex103through the computer ex111. The coding processes may be performed by thecamera ex116, the computer ex111, or the streaming server ex103, orshared among them.

Furthermore, the coding and decoding processes may be performed by anLSI ex500 generally included in each of the computer ex111 and thedevices. The LSI ex500 may be configured of a single chip or a pluralityof chips. Software for coding and decoding video may be integrated intosome type of a recording medium (such as a CD-ROM, a flexible disk, anda hard disk) that is readable by the computer ex111 and others, and thecoding and decoding processes may be performed using the software.Furthermore, when the cellular phone ex114 is equipped with a camera,the image data obtained by the camera may be transmitted. The video datais data coded by the LSI ex500 included in the cellular phone ex114.

Furthermore, the streaming server ex103 may be composed of servers andcomputers, and may decentralize data and process the decentralized data,record, or distribute data.

As described above, the clients may receive and reproduce the coded datain the content providing system ex100. In other words, the clients canreceive and decode information transmitted by the user, and reproducethe decoded data in real time in the content providing system ex100, sothat the user who does not have any particular right and equipment canimplement personal broadcasting.

Aside from the example of the content providing system ex100, at leastone of the moving picture coding apparatus (the picture codingapparatus) and the moving picture decoding apparatus (the picturedecoding apparatus) described in each of Embodiments may be implementedin a digital broadcasting system ex200 illustrated in FIG. 18. Morespecifically, a broadcast station ex201 communicates or transmits, viaradio waves to a broadcast satellite ex202, multiplexed data obtained bymultiplexing audio data and others onto video data. The video data isdata coded by the moving picture coding method described in each ofEmbodiments (that is, the video data is data coded by the picture codingapparatus according to an implementation of the present invention). Uponreceipt of the multiplexed data, the broadcast satellite ex202 transmitsradio waves for broadcasting. Then, a home-use antenna ex204 with asatellite broadcast reception function receives the radio waves. Next, adevice such as a television (receiver) ex300 and a set top box (STB)ex217 decodes the received multiplexed data, and reproduces the decodeddata (that is, the system functions as the picture decoding apparatusaccording to an implementation of the present invention).

Furthermore, a reader/recorder ex218 (i) reads and decodes themultiplexed data recorded on a recording media ex215, such as a DVD anda BD, or (ii) codes video signals in the recording medium ex215, and insome cases, writes data obtained by multiplexing an audio signal on thecoded data. The reader/recorder ex218 can include the moving picturedecoding apparatus or the moving picture coding apparatus as shown ineach of Embodiments. In this case, the reproduced video signals aredisplayed on the monitor ex219, and can be reproduced by another deviceor system using the recording medium ex215 on which the multiplexed datais recorded. It is also possible to implement the moving picturedecoding apparatus in the set top box ex217 connected to the cable ex203for a cable television or to the antenna ex204 for satellite and/orterrestrial broadcasting, so as to display the video signals on themonitor ex219 of the television ex300. The moving picture decodingapparatus may be implemented not in the set top box but in thetelevision ex300.

FIG. 19 illustrates the television (receiver) ex300 that uses the movingpicture coding method and the moving picture decoding method describedin each of Embodiments. The television ex300 includes: a tuner ex301that obtains or provides multiplexed data obtained by multiplexing audiodata onto video data, through the antenna ex204 or the cable ex203, etc.that receives a broadcast; a modulation/demodulation unit ex302 thatdemodulates the received multiplexed data or modulates data intomultiplexed data to be supplied outside; and amultiplexing/demultiplexing unit ex303 that demultiplexes the modulatedmultiplexed data into video data and audio data, or multiplexes videodata and audio data coded by a signal processing unit ex306 into data.

The television ex300 further includes: a signal processing unit ex306including an audio signal processing unit ex304 and a video signalprocessing unit ex305 (functioning as the picture coding apparatus orthe picture decoding apparatus according to an implementation of thepresent invention) that decode audio data and video data and code audiodata and video data, respectively; and an output unit ex309 including aspeaker ex307 that provides the decoded audio signal, and a display unitex308 that displays the decoded video signal, such as a display.Furthermore, the television ex300 includes an interface unit ex317including an operation input unit ex312 that receives an input of a useroperation. Furthermore, the television ex300 includes a control unitex310 that controls overall each constituent element of the televisionex300, and a power supply circuit unit ex311 that supplies power to eachof the elements. Other than the operation input unit ex312, theinterface unit ex317 may include: a bridge ex313 that is connected to anexternal device, such as the reader/recorder ex218; a slot unit ex314for enabling attachment of the recording medium ex216, such as an SDcard; a driver ex315 to be connected to an external recording medium,such as a hard disk; and a modem ex316 to be connected to a telephonenetwork. Here, the recording medium ex216 can electrically recordinformation using a non-volatile/volatile semiconductor memory elementfor storage. The constituent elements of the television ex300 areconnected to each other through a synchronous bus.

First, the configuration in which the television ex300 decodesmultiplexed data obtained from outside through the antenna ex204 andothers and reproduces the decoded data will be described. In thetelevision ex300, upon a user operation through a remote controllerex220 and others, the multiplexing/demultiplexing unit ex303demultiplexes the multiplexed data demodulated by themodulation/demodulation unit ex302, under control of the control unitex310 including a CPU. Furthermore, the audio signal processing unitex304 decodes the demultiplexed audio data, and the video signalprocessing unit ex305 decodes the demultiplexed video data, using thedecoding method described in each of Embodiments, in the televisionex300. The output unit ex309 provides the decoded video signal and audiosignal outside, respectively. When the output unit ex309 provides thevideo signal and the audio signal, the signals may be temporarily storedin buffers ex318 and ex319, and others so that the signals arereproduced in synchronization with each other. Furthermore, thetelevision ex300 may read multiplexed data not through a broadcast andothers but from the recording media ex215 and ex216, such as a magneticdisk, an optical disk, and a SD card. Next, a configuration in which thetelevision ex300 codes an audio signal and a video signal, and transmitsthe data outside or writes the data on a recording medium will bedescribed. In the television ex300, upon a user operation through theremote controller ex220 and others, the audio signal processing unitex304 codes an audio signal, and the video signal processing unit ex305codes a video signal, under control of the control unit ex310 using thecoding method described in each of Embodiments. Themultiplexing/demultiplexing unit ex303 multiplexes the coded videosignal and audio signal, and provides the resulting signal outside. Whenthe multiplexing/demultiplexing unit ex303 multiplexes the video signaland the audio signal, the signals may be temporarily stored in thebuffers ex320 and ex321, and others so that the signals are reproducedin synchronization with each other. Here, the buffers ex318, ex319,ex320, and ex321 may be plural as illustrated, or at least one buffermay be shared in the television ex300. Furthermore, although notillustrate, data may be stored in a buffer so that the system overflowand underflow may be avoided between the modulation/demodulation unitex302 and the multiplexing/demultiplexing unit ex303, for example.

Furthermore, the television ex300 may include a configuration forreceiving an AV input from a microphone or a camera other than theconfiguration for obtaining audio and video data from a broadcast or arecording medium, and may code the obtained data. Although thetelevision ex300 can code, multiplex, and provide outside data in thedescription, it may be capable of only receiving, decoding, andproviding outside data but not the coding, multiplexing, and providingoutside data.

Furthermore, when the reader/recorder ex218 reads or writes multiplexeddata from or on a recording medium, one of the television ex300 and thereader/recorder ex218 may decode or code the multiplexed data, and thetelevision ex300 and the reader/recorder ex218 may share the decoding orcoding.

As an example, FIG. 20 illustrates a configuration of an informationreproducing/recording unit ex400 when data is read or written from or onan optical disk. The information reproducing/recording unit ex400includes constituent elements ex401, ex402, ex403, ex404, ex405, ex406,and ex407 to be described hereinafter. The optical head ex401 irradiatesa laser spot in a recording surface of the recording medium ex215 thatis an optical disk to write information, and detects reflected lightfrom the recording surface of the recording medium ex215 to read theinformation. The modulation recording unit ex402 electrically drives asemiconductor laser included in the optical head ex401, and modulatesthe laser light according to recorded data. The reproductiondemodulating unit ex403 amplifies a reproduction signal obtained byelectrically detecting the reflected light from the recording surfaceusing a photo detector included in the optical head ex401, anddemodulates the reproduction signal by separating a signal componentrecorded on the recording medium ex215 to reproduce the necessaryinformation. The buffer ex404 temporarily holds the information to berecorded on the recording medium ex215 and the information reproducedfrom the recording medium ex215. The disk motor ex405 rotates therecording medium ex215. The servo control unit ex406 moves the opticalhead ex401 to a predetermined information track while controlling therotation drive of the disk motor ex405 so as to follow the laser spot.The system control unit ex407 controls overall the informationreproducing/recording unit ex400. The reading and writing processes canbe implemented by the system control unit ex407 using variousinformation stored in the buffer ex404 and generating and adding newinformation as necessary, and by the modulation recording unit ex402,the reproduction demodulating unit ex403, and the servo control unitex406 that record and reproduce information through the optical headex401 while being operated in a coordinated manner. The system controlunit ex407 includes, for example, a microprocessor, and executesprocessing by causing a computer to execute a program for read andwrite.

Although the optical head ex401 irradiates a laser spot in thedescription, it may perform high-density recording using near fieldlight.

FIG. 21 illustrates the recording medium ex215 that is the optical disk.On the recording surface of the recording medium ex215, guide groovesare spirally formed, and an information track ex230 records, in advance,address information indicating an absolute position on the diskaccording to change in a shape of the guide grooves. The addressinformation includes information for determining positions of recordingblocks ex231 that are a unit for recording data. Reproducing theinformation track ex230 and reading the address information in anapparatus that records and reproduces data can lead to determination ofthe positions of the recording blocks. Furthermore, the recording mediumex215 includes a data recording area ex233, an inner circumference areaex232, and an outer circumference area ex234. The data recording areaex233 is an area for use in recording the user data. The innercircumference area ex232 and the outer circumference area ex234 that areinside and outside of the data recording area ex233, respectively arefor specific use except for recording the user data. The informationreproducing/recording unit 400 reads and writes coded audio, coded videodata, or multiplexed data obtained by multiplexing the coded audio andvideo data, from and on the data recording area ex233 of the recordingmedium ex215.

Although an optical disk having a layer, such as a DVD and a BD isdescribed as an example in the description, the optical disk is notlimited to such, and may be an optical disk having a multilayerstructure and capable of being recorded on a part other than thesurface. Furthermore, the optical disk may have a structure formultidimensional recording/reproduction, such as recording ofinformation using light of colors with different wavelengths in the sameportion of the optical disk and for recording information havingdifferent layers from various angles.

Furthermore, a car ex210 having an antenna ex205 can receive data fromthe satellite ex202 and others, and reproduce video on a display devicesuch as a car navigation system ex211 set in the car ex210, in thedigital broadcasting system ex200. Here, a configuration of the carnavigation system ex211 will be a configuration, for example, includinga GPS receiving unit from the configuration illustrated in FIG. 19. Thesame will be true for the configuration of the computer ex111, thecellular phone ex114, and others.

FIG. 22A illustrates the cellular phone ex114 that uses the movingpicture coding method and the moving picture decoding method describedin Embodiments. The cellular phone ex114 includes: an antenna ex350 fortransmitting and receiving radio waves through the base station ex110; acamera unit ex365 capable of capturing moving and still images; and adisplay unit ex358 such as a liquid crystal display for displaying thedata such as decoded video captured by the camera unit ex365 or receivedby the antenna ex350. The cellular phone ex114 further includes: a mainbody unit including an operation key unit ex366; an audio output unitex357 such as a speaker for output of audio; an audio input unit ex356such as a microphone for input of audio; a memory unit ex367 for storingcaptured video or still pictures, recorded audio, coded or decoded dataof the received video, the still pictures, e-mails, or others; and aslot unit ex364 that is an interface unit for a recording medium thatstores data in the same manner as the memory unit ex367.

Next, an example of a configuration of the cellular phone ex114 will bedescribed with reference to FIG. 22B. In the cellular phone ex114, amain control unit ex360 designed to control overall each unit of themain body including the display unit ex358 as well as the operation keyunit ex366 is connected mutually, via a synchronous bus ex370, to apower supply circuit unit ex361, an operation input control unit ex362,a video signal processing unit ex355, a camera interface unit ex363, aliquid crystal display (LCD) control unit ex359, amodulation/demodulation unit ex352, a multiplexing/demultiplexing unitex353, an audio signal processing unit ex354, the slot unit ex364, andthe memory unit ex367.

When a call-end key or a power key is turned ON by a user's operation,the power supply circuit unit ex361 supplies the respective units withpower from a battery pack so as to activate the cell phone ex114.

In the cellular phone ex114, the audio signal processing unit ex354converts the audio signals collected by the audio input unit ex356 invoice conversation mode into digital audio signals under the control ofthe main control unit ex360 including a CPU, ROM, and RAM. Then, themodulation/demodulation unit ex352 performs spread spectrum processingon the digital audio signals, and the transmitting and receiving unitex351 performs digital-to-analog conversion and frequency conversion onthe data, so as to transmit the resulting data via the antenna ex350.Also, in the cellular phone ex114, the transmitting and receiving unitex351 amplifies the data received by the antenna ex350 in voiceconversation mode and performs frequency conversion and theanalog-to-digital conversion on the data. Then, themodulation/demodulation unit ex352 performs inverse spread spectrumprocessing on the data, and the audio signal processing unit ex354converts it into analog audio signals, so as to output them via theaudio output unit ex357.

Furthermore, when an e-mail in data communication mode is transmitted,text data of the e-mail inputted by operating the operation key unitex366 and others of the main body is sent out to the main control unitex360 via the operation input control unit ex362. The main control unitex360 causes the modulation/demodulation unit ex352 to perform spreadspectrum processing on the text data, and the transmitting and receivingunit ex351 performs the digital-to-analog conversion and the frequencyconversion on the resulting data to transmit the data to the basestation ex110 via the antenna ex350. When an e-mail is received,processing that is approximately inverse to the processing fortransmitting an e-mail is performed on the received data, and theresulting data is provided to the display unit ex358.

When video, still images, or video and audio in data communication modeis or are transmitted, the video signal processing unit ex355 compressesand codes video signals supplied from the camera unit ex365 using themoving picture coding method shown in each of Embodiments (that is, thevideo signal processing unit ex355 functions as the picture codingapparatus according to an implementation of the present invention), andtransmits the coded video data to the multiplexing/demultiplexing unitex353. In contrast, during when the camera unit ex365 captures video,still images, and others, the audio signal processing unit ex354 codesaudio signals collected by the audio input unit ex356, and transmits thecoded audio data to the multiplexing/demultiplexing unit ex353.

The multiplexing/demultiplexing unit ex353 multiplexes the coded videodata supplied from the video signal processing unit ex355 and the codedaudio data supplied from the audio signal processing unit ex354, using apredetermined method. Then, the modulation/demodulation unit (themodulation/demodulation circuit unit) ex352 performs spread spectrumprocessing on the multiplexed data, and the transmitting and receivingunit ex351 performs digital-to-analog conversion and frequencyconversion on the data so as to transmit the resulting data via theantenna ex350.

When receiving data of a video file which is linked to a Web page andothers in data communication mode or when receiving an e-mail with videoand/or audio attached, in order to decode the multiplexed data receivedvia the antenna ex350, the multiplexing/demultiplexing unit ex353demultiplexes the multiplexed data into a video data bit stream and anaudio data bit stream, and supplies the video signal processing unitex355 with the coded video data and the audio signal processing unitex354 with the coded audio data, through the synchronous bus ex370. Thevideo signal processing unit ex355 decodes the video signal using amoving picture decoding method corresponding to the coding method shownin each of Embodiments (that is, the video signal processing unit ex355functions as the picture decoding apparatus according to an aspect ofthe present invention), and then the display unit ex358 displays, forinstance, the video and still images included in the video file linkedto the Web page via the LCD control unit ex359. Furthermore, the audiosignal processing unit ex354 decodes the audio signal, and the audiooutput unit ex357 provides the audio.

Furthermore, similarly to the television ex300, a terminal such as thecellular phone ex114 probably have 3 types of implementationconfigurations including not only (i) a transmitting and receivingterminal including both a coding apparatus and a decoding apparatus, butalso (ii) a transmitting terminal including only a coding apparatus and(iii) a receiving terminal including only a decoding apparatus. Althoughthe digital broadcasting system ex200 receives and transmits themultiplexed data obtained by multiplexing audio data onto video data inthe description, the multiplexed data may be data obtained bymultiplexing not audio data but character data related to video ontovideo data, and may be not multiplexed data but video data itself.

As such, the moving picture coding method and the moving picturedecoding method in each of Embodiments can be used in any of the devicesand systems described. Thus, the advantages described in each ofEmbodiments can be obtained.

Furthermore, the present invention is not limited to Embodiments, andvarious modifications and revisions are possible without departing fromthe scope of the present invention.

Embodiment 7

Video data can be generated by switching, as necessary, between (i) themoving picture coding method or the moving picture coding apparatusshown in each of Embodiments and (ii) a moving picture coding method ora moving picture coding apparatus in conformity with a differentstandard, such as MPEG-2, MPEG4-AVC, and VC-1.

Here, when a plurality of video data that conforms to the differentstandards is generated and is then decoded, the decoding methods need tobe selected to conform to the different standards. However, since towhich standard each of the plurality of the video data to be decodedconforms cannot be detected, there is a problem that an appropriatedecoding method cannot be selected.

In order to solve the problem, multiplexed data obtained by multiplexingaudio data and others onto video data has a structure includingidentification information indicating to which standard the video dataconforms. The specific structure of the multiplexed data including thevideo data generated in the moving picture coding method and by themoving picture coding apparatus shown in each of Embodiments will behereinafter described. The multiplexed data is a digital stream in theMPEG2-Transport Stream format.

FIG. 23 illustrates a structure of the multiplexed data. As illustratedin FIG. 23, the multiplexed data can be obtained by multiplexing atleast one of a video stream, an audio stream, a presentation graphicsstream (PG), and an interactive graphics stream. The video streamrepresents primary video and secondary video of a movie, the audiostream (IG) represents a primary audio part and a secondary audio partto be mixed with the primary audio part, and the presentation graphicsstream represents subtitles of the movie. Here, the primary video isnormal video to be displayed on a screen, and the secondary video isvideo to be displayed on a smaller window in the primary video.Furthermore, the interactive graphics stream represents an interactivescreen to be generated by arranging the GUI components on a screen. Thevideo stream is coded in the moving picture coding method or by themoving picture coding apparatus shown in each of Embodiments, or in amoving picture coding method or by a moving picture coding apparatus inconformity with a conventional standard, such as MPEG-2, MPEG4-AVC, andVC-1. The audio stream is coded in accordance with a standard, such asDolby-AC-3, Dolby Digital Plus, MLP, DTS, DTS-HD, and linear PCM.

Each stream included in the multiplexed data is identified by PID. Forexample, 0x1011 is allocated to the video stream to be used for video ofa movie, 0x1100 to 0x111F are allocated to the audio streams, 0x1200 to0x121F are allocated to the presentation graphics streams, 0x1400 to0x141F are allocated to the interactive graphics streams, 0x1B00 to0x1B1F are allocated to the video streams to be used for secondary videoof the movie, and 0x1A00 to 0x1A1F are allocated to the audio streams tobe used for the secondary video to be mixed with the primary audio.

FIG. 24 schematically illustrates how data is multiplexed. First, avideo stream ex235 composed of video frames and an audio stream ex238composed of audio frames are transformed into a stream of PES packetsex236 and a stream of PES packets ex239, and further into TS packetsex237 and TS packets ex240, respectively. Similarly, data of apresentation graphics stream ex241 and data of an interactive graphicsstream ex244 are transformed into a stream of PES packets ex242 and astream of PES packets ex245, and further into TS packets ex243 and TSpackets ex246, respectively. These TS packets are multiplexed into astream to obtain multiplexed data ex247.

FIG. 25 illustrates how a video stream is stored in a stream of PESpackets in more detail. The first bar in FIG. 25 shows a video framestream in a video stream. The second bar shows the stream of PESpackets. As indicated by arrows denoted as yy1, yy2, yy3, and yy4 inFIG. 25, the video stream is divided into pictures as I-pictures,B-pictures, and P-pictures each of which is a video presentation unit,and the pictures are stored in a payload of each of the PES packets.Each of the PES packets has a PES header, and the PES header stores aPresentation Time-Stamp (PTS) indicating a display time of the picture,and a Decoding Time-Stamp (DTS) indicating a decoding time of thepicture.

FIG. 26 illustrates a format of TS packets to be finally written on themultiplexed data. Each of the TS packets is a 188-byte fixed lengthpacket including a 4-byte TS header having information, such as a PIDfor identifying a stream and a 184-byte TS payload for storing data. ThePES packets are divided, and stored in the TS payloads, respectively.When a BD ROM is used, each of the TS packets is given a 4-byteTP_Extra_Header, thus resulting in 192-byte source packets. The sourcepackets are written on the multiplexed data. The TP_Extra_Header storesinformation such as an Arrival_Time_Stamp (ATS). The ATS shows atransfer start time at which each of the TS packets is to be transferredto a PID filter. The source packets are arranged in the multiplexed dataas shown at the bottom of FIG. 26. The numbers incrementing from thehead of the multiplexed data are called source packet numbers (SPNs).

Each of the TS packets included in the multiplexed data includes notonly streams of audio, video, subtitles and others, but also a ProgramAssociation Table (PAT), a Program Map Table (PMT), and a Program ClockReference (PCR). The PAT shows what a PID in a PMT used in themultiplexed data indicates, and a PID of the PAT itself is registered aszero. The PMT stores PIDs of the streams of video, audio, subtitles andothers included in the multiplexed data, and attribute information ofthe streams corresponding to the PIDs. The PMT also has variousdescriptors relating to the multiplexed data. The descriptors haveinformation such as copy control information showing whether copying ofthe multiplexed data is permitted or not. The PCR stores STC timeinformation corresponding to an ATS showing when the PCR packet istransferred to a decoder, in order to achieve synchronization between anArrival Time Clock (ATC) that is a time axis of ATSs, and an System TimeClock (STC) that is a time axis of PTSs and DTSs.

FIG. 27 illustrates the data structure of the PMT in detail. A PMTheader is disposed at the top of the PMT. The PMT header describes thelength of data included in the PMT and others. A plurality ofdescriptors relating to the multiplexed data is disposed after the PMTheader. Information such as the copy control information is described inthe descriptors. After the descriptors, a plurality of pieces of streaminformation relating to the streams included in the multiplexed data isdisposed. Each piece of stream information includes stream descriptorseach describing information, such as a stream type for identifying acompression codec of a stream, a stream PID, and stream attributeinformation (such as a frame rate or an aspect ratio). The streamdescriptors are equal in number to the number of streams in themultiplexed data.

When the multiplexed data is recorded on a recording medium and others,it is recorded together with multiplexed data information files.

Each of the multiplexed data information files is management informationof the multiplexed data as shown in FIG. 28. The multiplexed datainformation files are in one to one correspondence with the multiplexeddata, and each of the files includes multiplexed data information,stream attribute information, and an entry map.

As illustrated in FIG. 28, the multiplexed data includes a system rate,a reproduction start time, and a reproduction end time. The system rateindicates the maximum transfer rate at which a system target decoder tobe described later transfers the multiplexed data to a PID filter. Theintervals of the ATSs included in the multiplexed data are set to nothigher than a system rate. The reproduction start time indicates a PTSin a video frame at the head of the multiplexed data. An interval of oneframe is added to a PTS in a video frame at the end of the multiplexeddata, and the PTS is set to the reproduction end time.

As shown in FIG. 29, a piece of attribute information is registered inthe stream attribute information, for each PID of each stream includedin the multiplexed data. Each piece of attribute information hasdifferent information depending on whether the corresponding stream is avideo stream, an audio stream, a presentation graphics stream, or aninteractive graphics stream. Each piece of video stream attributeinformation carries information including what kind of compression codecis used for compressing the video stream, and the resolution, aspectratio and frame rate of the pieces of picture data that is included inthe video stream. Each piece of audio stream attribute informationcarries information including what kind of compression codec is used forcompressing the audio stream, how many channels are included in theaudio stream, which language the audio stream supports, and how high thesampling frequency is. The video stream attribute information and theaudio stream attribute information are used for initialization of adecoder before the player plays back the information.

In Embodiment 7, the multiplexed data to be used is of a stream typeincluded in the PMT. Furthermore, when the multiplexed data is recordedon a recording medium, the video stream attribute information includedin the multiplexed data information is used. More specifically, themoving picture coding method or the moving picture coding apparatusdescribed in each of Embodiments includes a step or a unit forallocating unique information indicating video data generated by themoving picture coding method or the moving picture coding apparatus ineach of Embodiments, to the stream type included in the PMT or the videostream attribute information. With the configuration, the video datagenerated by the moving picture coding method or the moving picturecoding apparatus described in each of Embodiments can be distinguishedfrom video data that conforms to another standard.

Furthermore, FIG. 30 illustrates steps of the moving picture decodingmethod according to Embodiment 7. In Step exS100, the stream typeincluded in the PMT or the video stream attribute information isobtained from the multiplexed data. Next, in Step exS101, it isdetermined whether or not the stream type or the video stream attributeinformation indicates that the multiplexed data is generated by themoving picture coding method or the moving picture coding apparatus ineach of Embodiments. When it is determined that the stream type or thevideo stream attribute information indicates that the multiplexed datais generated by the moving picture coding method or the moving picturecoding apparatus in each of Embodiments, in Step exS102, decoding isperformed by the moving picture decoding method in each of Embodiments.Furthermore, when the stream type or the video stream attributeinformation indicates conformance to the conventional standards, such asMPEG-2, MPEG4-AVC, and VC-1, in Step exS103, decoding is performed by amoving picture decoding method in conformity with the conventionalstandards.

As such, allocating a new unique value to the stream type or the videostream attribute information enables determination whether or not themoving picture decoding method or the moving picture decoding apparatusthat is described in each of Embodiments can perform decoding. Even whenmultiplexed data that conforms to a different standard, an appropriatedecoding method or apparatus can be selected. Thus, it becomes possibleto decode information without any error. Furthermore, the moving picturecoding method or apparatus, or the moving picture decoding method orapparatus in Embodiment 7 can be used in the devices and systemsdescribed above.

Embodiment 8

Each of the moving picture coding method, the moving picture codingapparatus, the moving picture decoding method, and the moving picturedecoding apparatus in each of Embodiments is typically achieved in theform of an integrated circuit or a Large Scale Integrated (LSI) circuit.As an example of the LSI, FIG. 31 illustrates a configuration of the LSIex500 that is made into one chip. The LSI ex500 includes elements ex501,ex502, ex503, ex504, ex505, ex506, ex507, ex508, and ex509 to bedescribed below, and the elements are connected to each other through abus ex510. The power supply circuit unit ex505 is activated by supplyingeach of the elements with power when the power supply circuit unit ex505is turned on.

For example, when coding is performed, the LSI ex500 receives an AVsignal from a microphone ex117, a camera ex113, and others through an AVIO ex509 under control of a control unit ex501 including a CPU ex502, amemory controller ex503, a stream controller ex504, and a drivingfrequency control unit ex512. The received AV signal is temporarilystored in an external memory ex511, such as an SDRAM. Under control ofthe control unit ex501, the stored data is segmented into data portionsaccording to the processing amount and speed to be transmitted to asignal processing unit ex507. Then, the signal processing unit ex507codes an audio signal and/or a video signal. Here, the coding of thevideo signal is the coding described in each of Embodiments.Furthermore, the signal processing unit ex507 sometimes multiplexes thecoded audio data and the coded video data, and a stream IO ex506provides the multiplexed data outside. The provided multiplexed data istransmitted to the base station ex107, or written on the recording mediaex215. When data sets are multiplexed, the data should be temporarilystored in the buffer ex508 so that the data sets are synchronized witheach other.

Although the memory ex511 is an element outside the LSI ex500, it may beincluded in the LSI ex500. The buffer ex508 is not limited to onebuffer, but may be composed of buffers. Furthermore, the LSI ex500 maybe made into one chip or a plurality of chips.

Furthermore, although the control unit ex510 includes the CPU ex502, thememory controller ex503, the stream controller ex504, the drivingfrequency control unit ex512, the configuration of the control unitex510 is not limited to such. For example, the signal processing unitex507 may further include a CPU. Inclusion of another CPU in the signalprocessing unit ex507 can improve the processing speed. Furthermore, asanother example, the CPU ex502 may serve as or be a part of the signalprocessing unit ex507, and, for example, may include an audio signalprocessing unit. In such a case, the control unit ex501 includes thesignal processing unit ex507 or the CPU ex502 including a part of thesignal processing unit ex507.

The name used here is LSI, but it may also be called IC, system LSI,super LSI, or ultra LSI depending on the degree of integration.

Moreover, ways to achieve integration are not limited to the LSI, and aspecial circuit or a general purpose processor and so forth can alsoachieve the integration. Field Programmable Gate Array (FPGA) that canbe programmed after manufacturing LSIs or a reconfigurable processorthat allows re-configuration of the connection or configuration of anLSI can be used for the same purpose.

In the future, with advancement in semiconductor technology, a brand-newtechnology may replace LSI. The functional blocks can be integratedusing such a technology. The possibility is that the present inventionis applied to biotechnology.

Embodiment 9

When video data generated in the moving picture coding method or by themoving picture coding apparatus described in each of Embodiments isdecoded, compared to when video data that conforms to a conventionalstandard, such as MPEG-2, MPEG4-AVC, and VC-1 is decoded, the processingamount probably increases. Thus, the LSI ex500 needs to be set to adriving frequency higher than that of the CPU ex502 to be used whenvideo data in conformity with the conventional standard is decoded.However, when the driving frequency is set higher, there is a problemthat the power consumption increases.

In order to solve the problem, the moving picture decoding apparatus,such as the television ex300 and the LSI ex500 is configured todetermine to which standard the video data conforms, and switch betweenthe driving frequencies according to the determined standard. FIG. 32illustrates a configuration ex800 in Embodiment 9. A driving frequencyswitching unit ex803 sets a driving frequency to a higher drivingfrequency when video data is generated by the moving picture codingmethod or the moving picture coding apparatus described in each ofEmbodiments. Then, the driving frequency switching unit ex803 instructsa decoding processing unit ex801 that executes the moving picturedecoding method described in each of Embodiments to decode the videodata. When the video data conforms to the conventional standard, thedriving frequency switching unit ex803 sets a driving frequency to alower driving frequency than that of the video data generated by themoving picture coding method or the moving picture coding apparatusdescribed in each of Embodiments. Then, the driving frequency switchingunit ex803 instructs the decoding processing unit ex802 that conforms tothe conventional standard to decode the video data.

More specifically, the driving frequency switching unit ex803 includesthe CPU ex502 and the driving frequency control unit ex512 in FIG. 31.Here, each of the decoding processing unit ex801 that executes themoving picture decoding method described in each of Embodiments and thedecoding processing unit ex802 that conforms to the conventionalstandard corresponds to the signal processing unit ex507 in FIG. 29. TheCPU ex502 determines to which standard the video data conforms. Then,the driving frequency control unit ex512 determines a driving frequencybased on a signal from the CPU ex502. Furthermore, the signal processingunit ex507 decodes the video data based on the signal from the CPUex502. For example, the identification information described inEmbodiment 7 is probably used for identifying the video data. Theidentification information is not limited to the one described inEmbodiment 7 but may be any information as long as the informationindicates to which standard the video data conforms. For example, whenwhich standard video data conforms to can be determined based on anexternal signal for determining that the video data is used for atelevision or a disk, etc., the determination may be made based on suchan external signal. Furthermore, the CPU ex502 selects a drivingfrequency based on, for example, a look-up table in which the standardsof the video data are associated with the driving frequencies as shownin FIG. 34. The driving frequency can be selected by storing the look-uptable in the buffer ex508 and in an internal memory of an LSI, and withreference to the look-up table by the CPU ex502.

FIG. 33 illustrates steps for executing a method in Embodiment 9. First,in Step exS200, the signal processing unit ex507 obtains identificationinformation from the multiplexed data. Next, in Step exS201, the CPUex502 determines whether or not the video data is generated by thecoding method and the coding apparatus described in each of Embodiments,based on the identification information. When the video data isgenerated by the coding method and the coding apparatus described ineach of Embodiments, in Step exS202, the CPU ex502 transmits a signalfor setting the driving frequency to a higher driving frequency to thedriving frequency control unit ex512. Then, the driving frequencycontrol unit ex512 sets the driving frequency to the higher drivingfrequency. On the other hand, when the identification informationindicates that the video data conforms to the conventional standard,such as MPEG-2, MPEG4-AVC, and VC-1, in Step exS203, the CPU ex502transmits a signal for setting the driving frequency to a lower drivingfrequency to the driving frequency control unit ex512. Then, the drivingfrequency control unit ex512 sets the driving frequency to the lowerdriving frequency than that in the case where the video data isgenerated by the moving picture coding method and the moving picturecoding apparatus described in each of Embodiment.

Furthermore, along with the switching of the driving frequencies, thepower conservation effect can be improved by changing the voltage to beapplied to the LSI ex500 or an apparatus including the LSI ex500. Forexample, when the driving frequency is set lower, the voltage to beapplied to the LSI ex500 or the apparatus including the LSI ex500 isprobably set to a voltage lower than that in the case where the drivingfrequency is set higher.

Furthermore, when the processing amount for decoding is larger, thedriving frequency may be set higher, and when the processing amount fordecoding is smaller, the driving frequency may be set lower as themethod for setting the driving frequency. Thus, the setting method isnot limited to the ones described above. For example, when theprocessing amount for decoding video data in conformity with MPEG-AVC islarger than the processing amount for decoding video data generated bythe moving picture coding method and the moving picture coding apparatusdescribed in each of Embodiments, the driving frequency is probably setin reverse order to the setting described above.

Furthermore, the method for setting the driving frequency is not limitedto the method for setting the driving frequency lower. For example, whenthe identification information indicates that the video data isgenerated by the moving picture coding method and the moving picturecoding apparatus described in each of Embodiments, the voltage to beapplied to the LSI ex500 or the apparatus including the LSI ex500 isprobably set higher. When the identification information indicates thatthe video data conforms to the conventional standard, such as MPEG-2,MPEG4-AVC, and VC-1, the voltage to be applied to the LSI ex500 or theapparatus including the LSI ex500 is probably set lower. As anotherexample, when the identification information indicates that the videodata is generated by the moving picture coding method and the movingpicture coding apparatus described in each of Embodiments, the drivingof the CPU ex502 does not probably have to be suspended. When theidentification information indicates that the video data conforms to theconventional standard, such as MPEG-2, MPEG4-AVC, and VC-1, the drivingof the CPU ex502 is probably suspended at a given time because the CPUex502 has extra processing capacity. Even when the identificationinformation indicates that the video data is generated by the movingpicture coding method and the moving picture coding apparatus describedin each of Embodiments, in the case where the CPU ex502 has extraprocessing capacity, the driving of the CPU ex502 is probably suspendedat a given time. In such a case, the suspending time is probably setshorter than that in the case where when the identification informationindicates that the video data conforms to the conventional standard,such as MPEG-2, MPEG4-AVC, and VC-1.

Accordingly, the power conservation effect can be improved by switchingbetween the driving frequencies in accordance with the standard to whichthe video data conforms. Furthermore, when the LSI ex500 or theapparatus including the LSI ex500 is driven using a battery, the batterylife can be extended with the power conservation effect.

Embodiment 10

There are cases where a plurality of video data that conforms todifferent standards, is provided to the devices and systems, such as atelevision and a mobile phone. In order to enable decoding the pluralityof video data that conforms to the different standards, the signalprocessing unit ex507 of the LSI ex500 needs to conform to the differentstandards. However, the problems of increase in the scale of the circuitof the LSI ex500 and increase in the cost arise with the individual useof the signal processing units ex507 that conform to the respectivestandards.

In order to solve the problem, what is conceived is a configuration inwhich the decoding processing unit for implementing the moving picturedecoding method described in each of Embodiments and the decodingprocessing unit that conforms to the conventional standard, such asMPEG-2, MPEG4-AVC, and VC-1 are partly shared. Ex900 in FIG. 35A showsan example of the configuration. For example, the moving picturedecoding method described in each of Embodiments and the moving picturedecoding method that conforms to MPEG4-AVC have, partly in common, thedetails of processing, such as entropy coding, inverse quantization,deblocking filtering, and motion compensated prediction. The details ofprocessing to be shared probably include use of a decoding processingunit ex902 that conforms to MPEG4-AVC. In contrast, a dedicated decodingprocessing unit ex901 is probably used for other processing that doesnot conform to MPEG4-AVC and is unique to the present invention. Sincethe present invention is characterized by a transformation unit inparticular, for example, the dedicated decoding processing unit ex901 isused for inverse transform. Otherwise, the decoding processing unit isprobably shared for one of the entropy coding, deblocking filtering, andmotion compensated prediction, or all of the processing. The decodingprocessing unit for implementing the moving picture decoding methoddescribed in each of Embodiments may be shared for the processing to beshared, and a dedicated decoding processing unit may be used forprocessing unique to that of MPEG4-AVC.

Furthermore, ex1000 in FIG. 35B shows another example in that processingis partly shared. This example uses a configuration including adedicated decoding processing unit ex1001 that supports the processingunique to the present invention, a dedicated decoding processing unitex1002 that supports the processing unique to another conventionalstandard, and a decoding processing unit ex1003 that supports processingto be shared between the moving picture decoding method in the presentinvention and the conventional moving picture decoding method. Here, thededicated decoding processing units ex1001 and ex1002 are notnecessarily specialized for the processing of the present invention andthe processing of the conventional standard, respectively, and may bethe ones capable of implementing general processing. Furthermore, theconfiguration of Embodiment 10 can be implemented by the LSI ex500.

As such, reducing the scale of the circuit of an LSI and reducing thecost are possible by sharing the decoding processing unit for theprocessing to be shared between the moving picture decoding method inthe present invention and the moving picture decoding method inconformity with the conventional standard.

INDUSTRIAL APPLICABILITY

The motion vector calculation method, the picture coding method, and thepicture decoding method according to the present invention produce aneffect of attaining a higher compression rate and are applicable to, forexample, video cameras, cellular phones with functions of capturing andreproducing video, personal computers, or recoding/playback devices.

REFERENCE SIGNS LIST

-   100 Picture coding apparatus-   101 Substractor-   102 Orthogonal transform unit-   103 Quantization unit-   104 Inverse quantization unit-   105 Inverse orthogonal transform unit-   106 Adder-   107 Block memory-   108 Frame memory-   109 Intra prediction unit-   110 Inter prediction unit-   111 Inter prediction control unit-   112 Picture type determination unit-   113 Temporal direct vector calculation unit-   114 Co-located reference direction determination unit-   115 Variable-length coding unit-   200 Picture decoding apparatus-   204 Inverse quantization unit-   205 Inverse orthogonal transform unit-   206 Adder-   207 Block memory-   208 Frame memory-   209 Intra prediction unit-   210 Inter prediction unit-   211 Inter prediction control unit-   213 Temporal direct vector calculation unit-   215 Variable-length decoding unit

The invention claimed is:
 1. A decoding method of decoding a currentblock included in a current picture, the current picture being includedin a coded video stream, the decoding method comprising: determining areference picture in the coded video stream, the reference picture beingincluded in one of (i) a first reference picture group of the currentblock and (ii) a second reference picture group of the current block;selecting a reference motion vector among from one or more referencemotion vectors of a reference block in the reference picture such thatin situation (A) when the reference block has a first reference motionvector and a second reference motion vector that respectively correspondto the first reference picture group and the second reference picturegroup, (i) the first reference motion vector is selected when thereference picture is included in the second reference picture group and(ii) the second reference motion vector is selected when the referencepicture is included in the first reference picture group, and insituation (B) when the reference block has only one reference motionvector, the only reference motion vector is selected; deriving themotion vector of the current block using the selected one referencemotion vector; and decoding the current block using the derived motionvector.
 2. The decoding method according to claim 1, further comprising:obtaining a position flag added to the current picture including thecurrent block, wherein, in the selecting, in the case where thereference block has, as the first reference motion vector and the secondreference motion vector, a forward reference motion vector directedforward and a backward reference motion vector directed backward, theforward reference motion vector is selected from among the firstreference motion vector and the second reference motion vector when theposition flag indicates that the reference block is located after thecurrent block, and the backward reference motion vector is selected fromamong the first reference motion vector and the second reference motionvector when the position flag indicates that the reference block islocated before the current block.
 3. A decoding apparatus for decoding acurrent block included in a current picture, the current picture beingincluded in a coded video stream, the decoding apparatus comprising: aprocessor; a non-transitory memory having stored thereon executableinstructions, which when executed by the processor, cause the processorto perform the following steps: determining a reference picture in thecoded video stream, the reference picture being included in one of (i) afirst reference picture group of the current block and (ii) a secondreference picture group of the current block; selecting a referencemotion vector among from one or more reference motion vectors of areference block in the reference picture such that in situation (A) whenthe reference block has a first reference motion vector and a secondreference motion vector that respectively correspond to the firstreference picture group and the second reference picture group, (i) thefirst reference motion vector is selected when the reference picture isincluded in the second reference picture group and (iii) the secondreference motion vector is selected when the reference picture isincluded in the first reference picture group, and in situation (B) whenthe reference block has only one reference motion vector, the onlyreference motion vector is selected; deriving the motion vector of thecurrent block using the selected one reference motion vector; anddecoding the current block using the derived motion vector.
 4. Thedecoding apparatus in claim 3, wherein: in the situation (A), the firstreference motion vector is a forward reference motion vector directedforward and the second reference motion vector is a backward referencemotion vector directed backward, the forward reference motion vector isselected when the reference block is located after the current block,and the backward reference motion vector is selected when the referenceblock is located before the current block.
 5. The decoding apparatus inclaim 3, wherein the non-transitory memory includes executableinstructions, which when executed by the processor, cause the processorto perform the following: obtaining a position flag added to the currentpicture including the current block, wherein, in said selecting, in thecase where the reference block has, as the first reference motion vectorand the second reference motion vector, a forward reference motionvector directed forward and a backward reference motion vector directedbackward, the forward reference motion vector is selected from among thefirst reference motion vector and the second reference motion vectorwhen the position flag indicates that the reference block is locatedafter the current block, and the backward reference motion vector isselected from among the first reference motion vector and the secondreference motion vector when the position flag indicates that thereference block is located before the current block.
 6. The decodingmethod according to claim 1, wherein: in the situation (A), the firstreference motion vector is a forward reference motion vector directedforward and the second reference motion vector is a backward referencemotion vector directed backward, the forward reference motion vector isselected when the reference block is located after the current block,and the backward reference motion vector is selected when the referenceblock is located before the current block.
 7. A decoding apparatus fordecoding a current block included in a current picture, the currentpicture being included in a coded video stream, the decoding methodcomprising: processing circuitry configured to: determine a referencepicture in the coded video stream, the reference picture being includedin one of (i) a first reference picture group of the current block and(ii) a second reference picture group of the current block; select areference motion vector among from one or more reference motion vectorsof a reference block in the reference picture such that in situation (A)when the reference block has a first reference motion vector and asecond reference motion vector that respectively correspond to the firstreference picture group and the second reference picture group, (i) thefirst reference motion vector is selected when the reference picture isincluded in the second reference picture group and (ii) the secondreference motion vector is selected when the reference picture isincluded in the first reference picture group, and in situation (B) whenthe reference block has only one reference motion vector, the onlyreference motion vector is selected; derive the motion vector of thecurrent block using the selected one reference motion vector; anddecoding circuitry configured to decode the current block using thederived motion vector.
 8. The decoding apparatus in claim 7, wherein: inthe situation (A), the first reference motion vector is a forwardreference motion vector directed forward and the second reference motionvector is a backward reference motion vector directed backward, theforward reference motion vector is selected when the reference block islocated after the current block, and the backward reference motionvector is selected when the reference block is located before thecurrent block.
 9. The decoding apparatus in claim 7, wherein thenon-transitory memory includes executable instructions, which whenexecuted by the processor, cause the processor to perform the following:obtaining a position flag added to the current picture including thecurrent block, wherein, in said selecting, in the case where thereference block has, as the first reference motion vector and the secondreference motion vector, a forward reference motion vector directedforward and a backward reference motion vector directed backward, theforward reference motion vector is selected from among the firstreference motion vector and the second reference motion vector when theposition flag indicates that the reference block is located after thecurrent block, and the backward reference motion vector is selected fromamong the first reference motion vector and the second reference motionvector when the position flag indicates that the reference block islocated before the current block.